Android: migrate external texture to SurfaceProducer (fix transparency under Impeller)

[frenradar]

Problem

On Android, the plugin creates the Filament external texture via the legacy
TextureRegistry.createSurfaceTexture(). Under Impeller (now the default
Android renderer), that legacy SurfaceTexture external-texture path composites
a transparent Filament view (View.BlendMode.TRANSLUCENT over a
CONFIG_TRANSPARENT swapchain, which createView already sets up) with
incorrect premultiplied alpha — so the transparent regions of the render read as
a grey/dark veil over whatever is composited beneath the Texture widget.
iOS/Metal is unaffected.

Minimal repro: a ViewerWidget with background: Color(0x00000000) layered over
other content. The transparent area veils grey on Android with Impeller on;
correct with Impeller off (confirms it's the external-texture composite path,
not the Filament view config).

Fix

Migrate the Android texture from createSurfaceTexture() to the modern
TextureRegistry.createSurfaceProducer() (setSize / getSurface / id /
release), which composites premultiplied alpha correctly with Impeller on. The
native-window handoff (getNativeWindowFromSurface) and everything else are
unchanged; this is Android-only.

Compatibility

SurfaceProducer landed in Flutter 3.22. thermion_flutter's min Flutter is
>=3.23.0-0.1.pre, so createSurfaceProducer() is always available in the
supported range — no version guard needed.

Testing

Device-verified on a Samsung Galaxy A15 (Android 14) rendering a transparent
Filament model over a map basemap: the grey veil is gone with Impeller on and the
model renders correctly; survives a brief app background/resume.

Follow-up (not in this PR)

A SurfaceProducer.Callback (onSurfaceAvailable / onSurfaceCleanup) to
recreate the swapchain when Android destroys/recreates the surface under memory
pressure or long backgrounding. The common case (brief background) survives
without it in my testing; the callback would harden the worst case. Happy to add
it here or in a follow-up — let me know your preference.