Ensure onTypingHintChanged run on UiThread

RemoteInputConnectionImpl can be run on a different
thread. Dispatch onTypingHintChanged operation with the
ViewRootImpl's handler to ensure it runs on the UiThread.

Bug: 281720315
Test: presubmit
Merged-In: I2fb9c549da19ff01e7cc3fd8bfc1f9c19aa0f0a8
Change-Id: I45376816cf01065e270dc1a8e2f3d1d3e564e9cd