Make IMM#peekInstance() thread-safe

Somehow InputMethodManager#peekInstance() has not been thread-safe.
Although we basically do not care this deprecated method, but let's
just update it for consistency.

Bug: 115891476
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I56bf05f76a7939f004efefdd956776e533bae058
1 file changed