Migrate IME to handle back with OnBackInvokedDispatcher.

We currently close the IME by having the target application forward KEYCODE_BACK to the IME process through InputMethodManager#dispatchInputEvent and having the IME handle the keycode in InputMethodService#onKeyDown. When apps opt in to OnBackInvokedDispatcher API, we will not dispatch KEYCODE_BACK to apps anymore. Thus we need to migrate IME to the new API for it to close on back invocation.

This implementation forwards OnBackInvokedCallbacks from the IME process
to the app process. This is necessary because all callbacks need to
exist in the app process for them to be considered by hardware back keys. While back gestures go through WM to resolve callbacks from the focused window, hw keys are directly sent to the focused window's ViewRootImpl, bypassing server side back nav logic.

Bug: 228358882
Test: atest CtsInputMethodTestCases:KeyboardVisibilityControlTest
Test: atest CtsInputMethodTestCases:InputMethodServiceTest
Test: atest CtsInputMethodTestCases
Change-Id: Ie207b63b11a56c9b2173f26b734a27b13ebccc60