Make InputMethodManagerService#MSG_* private if possible

Message formats used in InputMethodManagerService#handleMessage() are
supposed to be internal details of InputMethodManagerService.
Allowing other classes to directly post messages to
InputMethodManagerService would make it difficult for us to track the
source code.

This CL makes InputMethodManagerService#MSG_* as private constants as
many as possible, which should have no behavior changes.

Bug: 192412909
Test: presubmit
Change-Id: Ifdde313c884b9ec10e45fa6449e7541217f5fb24