Make IInputMethodPrivilegedOperations to async (2/N)

-. Remove VoidResultCallback of reportStartInput
   and let it be truly asynchronous.
-. Rename this method to reportStartInputAsync.

Bug: 183587528
Test: atest CtsInputMethodTestCases
Change-Id: Ic8e7f888f78f7c536a9228db02a8b355555d7220