Introduce IInputContextInvoker for better readability

This is a purely mechanical refactoring with no no behavior change.

With this CL, InputConnectionWrapper no longer deals with boilerplate
code around IInputContext such as RemoteException handling.

Bug: 192412909
Test: atest CtsInputMethodTestCases
Change-Id: I0ea27e915a07699a4c124750266d84346fbd7bf6