Declare IMM#mIInputContext with its concrete type

This is a small refactoring that changes no semantics but just reduces
some boilerplate code in InputMethodManager.

We know that IMM#mIInputContext is guaranteed to be an instance of
IInputConnectionWrapper, we can reduce some boilerplate code by using
its concrete class when declaring it.

Using its concrete type would also helps the runtime and AOT compiler
do optimizations such as devirtualization because
IInputConnectionWrapper is already declared as a final class.

Other that such code reduction and plausible performance implications,
there should be no behavior change in this CL.

Bug: 192412909
Test: presubmit
Change-Id: I2ee22292feeb6c806d2949bc05f2780923597e1b