android: expose GPUI's accessibility tree to TalkBack - #25

GPUI 0.3.7 already builds an AccessKit tree and hands it to PlatformWindow::a11y_init / a11y_tree_update, but the Android window leaves both as no-ops, so TalkBack only sees one opaque surface. This PR connects them to accesskit_android's InjectingAdapter. That adapter installs an AccessibilityDelegate on the existing host View, so hosts don't need any Java changes.

It's one new module (src/android/accessibility.rs) plus two hooks in window.rs and two small changes in jni.rs, about 250 lines in total.

How it works

• a11y_init stores GPUI's callbacks and attaches the adapter to the host view. That's the content view on the host-driven path, where the SurfaceView lives, and the decor view on the android-activity path.
• a11y_tree_update forwards each tree to the adapter. The adapter posts its work to the UI thread itself.
• set_host_activity moves the adapter onto the new Activity's views when the Activity is recreated.
• Each window keeps its own callbacks. With one window per host Activity (android: one window per host, GL fallback, backend pinning and IME fixes #26), the window that becomes active takes the adapter over.
• On the android-activity path, hover events are now reported as unhandled. GPUI doesn't use hover on mobile, and TalkBack's touch exploration arrives as hover events, which the delegate on the decor view needs to see.

Two things worth flagging

• Version. accesskit_android 0.7.5 is the newest release built on accesskit 0.24, which is the version gpui-pre 0.3.7 uses. It can move to 0.9 when GPUI moves to accesskit 0.25.
• Workaround. 0.7.5 raises accessibility events without checking AccessibilityManager.isEnabled(). Android throws on the UI thread when no service is listening, so the app crashed as soon as the screen reader disconnected. This PR checks first. Once accessibility is off, it calls GPUI's deactivation callback and attaches a fresh adapter, so turning the screen reader back on activates GPUI again instead of reading a stale tree. 0.9 does the isEnabled() check internally, so most of this can go away with the upgrade.

Testing

Tested with the host-driven path in a small lab app on GPUI Kit 0.7.0.

OnePlus CPH2581 (Android 16), TalkBack. Touch exploration lands on the right controls, double-tap toggles a checkbox, and turning TalkBack off with the volume keys no longer crashes the app:

Redroid (Android 14), uiautomator dump. Every node's bounds drawn over the rendering. Kit's checkboxes, radios, switches and sliders come through as their native Android classes with labels and checked state:

android-activity (NativeActivity) path, same phone. I tested the same bridge ported to the NativeActivity example in itsbalamurali/gpui-mobile#69, because the lab app uses the host-driven path. That's where the hover fix above came from: before it, touch exploration never reached the delegate. With it, TalkBack reads the controls and double-tap toggles them. One limitation remains: Android doesn't draw its green focus ring, because NativeActivity takes over the window surface that the ring would be drawn on.

Happy to change the approach if you'd prefer something different, for example a Java-side AccessibilityNodeProvider in the host instead of the injected delegate.

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