Improve API annotation for InputMethodManager

Annotate null-ness for APIs and parameter for
InputMethodManager, IInputMethodManagerInvoker
and IInputMethodManager.aidl.

This is a annotation improvement without any user
visible changes.

Bug: 237959367
Test: presubmit
Change-Id: I426fb9ca9009f0f91a4d2d6a39094ea9c20671f7