Add a wrapper for IInputMethodPrivilegedOperations

This is a mechanical refactoring to split out boilerplate code around
IPCs from InputMethodManager to another file.

Bug: 114418674
Bug: 113177698
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I9ca251482867daea84c2777f74fd9b8a2b0f29cd