Remove redundant synchronized blocks

Unless it is also touching any internal field that needs to be
synchronized with InputMethodManager#mH, just making IPC to
InputMethodManagerService (IMMS) does not need any additional
serialization in the caller side.  IMMS itself already has internal
lock to serializate incoming operations.

Bug: 114418674
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I214d2c481c9fe862fbb9ec0759df2c798915b9f0