Make IInputMethodManager to oneway (1/N)

Isolate the Completable class from the CancellationGroup
class because not all Completable objects could be cancelled.
So we don’t need to always hold a final CancellationGroup in
Completable class. If the Completable object could be cancelled,
pass it as parameter when doing await.

Bug: 163453493
Test: Manual test with keyboard
Test: atest CtsInputMethodTestCases
Change-Id: If79810d5acc36b5a8c6a2c7cfcabeb4d0ebed197