fix(android): normalize MotionEvent deviceId so a gesture cannot split across two Unity input devices

[AhmedYAbbas]

Description

Fixes an Android input freeze where all 3D object interaction inside Unity goes
permanently silent after a drag, while UGUI keeps responding.

Root cause

CustomUnityPlayer.onTouchEvent only rewrote deviceId when it was exactly 0:

if (event.deviceId == 0) {
    val modifiedEvent = event.copy(deviceId = -1)
    ...
}

Android does not guarantee a stable deviceId across a single gesture in this
path. Measured on device, one drag arrived as DOWN(deviceId=5) followed by
UP(deviceId=0) with the same downTime. With the conditional rewrite, the
press went to Unity input device 5 and the release went to device -1, so
device 5's touch was never released. <Touchscreen>/Press latched pressed
forever and every action bound to it stopped firing, while <Pointer>-bound
UGUI was unaffected — which is why the symptom looks like "3D interaction broke
but the UI still works".

Fix

Rewrite deviceId unconditionally to a single NORMALIZED_DEVICE_ID = -1, so a
gesture's DOWN/MOVE/UP always land on one Unity input device. -1 is the
value the original Flutter workaround already used and is known to be picked up
by Unity's Input System.

Also fixed: double-recycle of the incoming event

The old code called event.recycle() on the event handed to onTouchEvent.
View.onTouchEvent does not take ownership — ViewRootImpl still owns that
event and recycles it after dispatch. Recycling here returned it to the pool
twice, so a later MotionEvent.obtain() could hand the same instance to two
owners. That was a real bug, though on-device measurement showed it was not
the cause of this freeze. The recycle() call is removed.

modifiedEvent is intentionally left unrecycled: Unity may retain it past the
call, and recycling it here would reintroduce a genuine use-after-free. An
un-recycled obtain() is only a missed pool return.

Scope

One file, Android only: android/.../CustomUnityPlayer.kt (+35 / -8). No public
API change, no Dart/iOS changes.