Deprecate InputMethodManager#getInstance()

With this CL, no one in the Framework is using
InputMethodManager#getInstance() directly or indirectly.  It is time
to mark this method deprecated.

For applications that still call InputMethodManager#getInstance()
directly or indirectly via reflection, they will start seeing warnings
with stacktrace in logcat.

Except for that explict warnings in logcat, there is no behavior
change in this CL.  Added a new test to make sure that
InputMethodManager#getInstance() and InputMethodManager#peekInstance()
are still working in a way we expected for such applications.

Fix: 115891476
Test: atest FrameworksCoreTests:android.view.inputmethod.InputMethodManagerTest
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Ib393086d921f91993395b5f0007b725a5db7bf22