[Android] Request unbuffered dispatch on MotionEvent.ACTION_MOVE for HCPP platform views - #192249

…latformViewGestureRecognizer` rejects gesture (flutter#192259)

Changes platform view touch input for all modes to request the
unbuffered stream for the given event when the GestureArena determines
the platform view has bowed out of ownership of that event, via
[View.requestUnbufferedDispatch()](https://developer.android.com/reference/kotlin/android/view/View#requestunbuffereddispatch).
Flutter content itself makes this request, but currently platform views
don't.

This greatly improves platform view scrolling smoothness, while having
no breaking change or impact on platform views performing gesture
handling (because the GestureArena has determined that they don't own
the input event).

### Problem & Invariant
Under high-load or scrolling scenarios, buffered touch dispatch causes
visible jank and input lag. Flutter requests unbuffered dispatch for its
own rendering, but platform views continue using buffered streams unless
requested. Simply requesting unbuffered dispatch unconditionally can
cause issues if gestures are still competing or owned by the platform
view. Scoping unbuffered dispatch to gestures that Flutter has won
ensures that platform views receive fast, unbuffered dispatch
specifically during Flutter gestures (like scrolling).

### Architecture & Gesture Correlation (`embedderId`)
Previous iterations attempted to correlate gestures across the channel
by matching synthetic `downTime` timestamps packed into
`PointerDownEvent.timeStamp`. However, resamplers or clock drift can
alter event timestamps.

Instead, this PR uses Flutter's canonical `embedderId` mechanism:
1. **Android Touch Processor**: When an Android `MotionEvent` arrives,
`AndroidTouchProcessor` tracks it using
`MotionEventTracker.track(event)` and populates
`packet.putLong(motionEventId)`, which the framework receives as
`PointerEvent.embedderId`.
2. **Framework Arena Resolution**: `_PlatformViewGestureRecognizer`
records the `embedderId` of the pointer-down event. When Flutter wins
the arena and calls `rejectGesture(pointer)`, the framework sends
`gestureId` (set to `embedderId`) over the `flutter/platform_views`
system channel.
3. **Engine Correlation (`MotionEventTracker.peek`)**:
- `PlatformViewsController` and `PlatformViewsController2` handle
`onRejectGesture(viewId, gestureId)`.
- The controllers query
`motionEventTracker.peek(MotionEventId.from(gestureId))` to look up the
native `MotionEvent` without consuming it from the tracker.
- From this `MotionEvent`, the native `downTime` is retrieved and passed
to `FlutterMutatorView` and `PlatformViewWrapper`.
- `PlatformViewGestureTracker` marks `flutterWonGesture = true` if the
`downTime` matches the current active gesture stream, and keeps calling
`requestUnbufferedDispatch()` for all remaining events in the gesture
until `ACTION_UP` or `ACTION_CANCEL`.
4. **Graceful Degradation / Fails Closed**:
- If `peek()` returns `null` (e.g. if the event was already popped or on
non-Android embedders), or if `gestureId` is 0 or unmatched, the
optimization degrades gracefully to standard buffered touch dispatch
without breaking input handling.

### Tests
- **Framework tests**:
`packages/flutter/test/rendering/platform_view_test.dart`
- Verifies `rejectGesture` passes the pointer's `embedderId` as
`gestureId`.
- Verifies multi-touch gestures correctly pass individual `embedderId`s.
  - Verifies no channel call is dispatched when `embedderId` is 0.
- **Engine Robolectric tests**:
- `PlatformViewsControllerTest`: Verifies `rejectGesture` with valid
`gestureId` looks up tracked `MotionEvent` and enables unbuffered
dispatch; verifies mismatched or missing IDs do not enable it.
- `PlatformViewsController2Test`: Verifies same behavior on the modern
hybrid composition pipeline.
- `FlutterMutatorViewTest` and `PlatformViewWrapperTest`: Verifies
gesture tracking across `ACTION_DOWN`, `ACTION_MOVE` sequences, and
resetting on `ACTION_UP`/`ACTION_CANCEL`.

Supersedes flutter#192249

## Pre-launch Checklist

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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
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- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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