Fix IME not dismissed with back gesture

Callback methods of callbacks registered by IME should be run on the same thread the ViewRootImpl holding IME's WindowOnBackInvokedDispatcher is created on.

Bug: 345427620
Test: Manual, i.e. verifying that IME is dismissed by back gesture without throwing IllegalStateException
Change-Id: I2883201aa59bb264e4c02246007c7c4788e8b07c