android: Use Android's scroll physics, and let hosts register an AssetSource

[tuyc]

Two gaps a host app hits on the android::host path. Both are Android-only:
everything here lives under src/android/, which lib.rs gates behind
#[cfg(target_os = "android")], so iOS does not compile it. Verified by
building the crate for aarch64-apple-ios with these changes applied.

Flings used iOS physics

AndroidPlatform never implemented Platform::gestures, so GPUI fell back to
GestureTuning::default() — whose scroll_physics is UIScrollView's
exponential decay. Every Android app built on gpui-mobile has therefore been
flinging with iOS physics: the content coasts much further than the platform's
own scroll views, which reads as the list not stopping where you expect.

This returns ScrollPhysics::android() instead, the AOSP OverScroller
friction spline that already lives in gestures.rs. GPUI flings in logical
pixels and a window's scale factor is Android's display density, so logical
pixels are density-independent pixels and the constructor's nominal 160 dpi
pairing is the right one.

Measured on a Huawei P40 (1080x2340 @3x), same conversation and gesture script,
counting frames for 2.5s after release:

fling	iOS physics	Android physics
swipe 80ms	71 frames	58 frames
swipe 300ms	60 frames	41 frames

Fewer frames because the spline sheds speed faster and the fling travels a
shorter distance — the point of the change, not a regression. Frame pacing is
unchanged: 16ms intervals throughout, droppedFrames = 0 in both.

The rest of ViewConfiguration (touch slop, tap timeouts) stays on GPUI's
defaults; reading the real values over JNI is a separate change.

A host could not install an asset source

host::start hands the launch closure a &mut App, which is one step too late
to install an asset source: Application::with_assets consumes the builder, and
the builder is created inside on_surface_created where the host cannot reach
it.

The failure is silent. AssetSource::load returning None is a valid answer,
so nothing logs and anything resolved through the asset source — icons, SVGs —
simply does not appear.

start_with_assets stores the configuration step in a static that
on_surface_created consumes when it builds the Application, right before
run_embedded. start keeps its current shape for hosts that do not need one.

Testing

• Huawei P40, android::host path, release build: flings and scrolling as
  above, no panics, droppedFrames/lateAcquireFrames both 0.
• cargo check --target aarch64-linux-android and
  cargo check --target aarch64-apple-ios both clean.