Reduce @SuppressWarnings("unchecked") a bit

This is a mechanical refactoring CL to follow up my previous CL [1],
which replaced our custom callback interfaces with AndroidFuture<T>,
which made us lose compile time checking in RemoteInputConnectionImpl.

Although we are still effectively annotating types manually, this CL
allows us to consolidate @SuppressWarnings("unchecked") into a single
utility method rather than scattering such annotations into multiple
methods.

Other than clean up the code, there should be no observable behavior
change in this CL.

 [1]: I74657826a99b11ca1f86932f8f41cca6e449cc8a
      b2d09de1

Bug: 192412909
Bug: 195699814
Test: presubmit
Change-Id: I59a60d7291142bf4827a05ce84183e84457c32b8