Make InputMethodManagerService#MSG_INITIALIZE_IME private

With this CL, all the message IDs defined as IMMS#MSG_* become private
constants.  Message formats used in

   InputMethodManagerService#handleMessage()

are now completely encapsulated within InputMethodManagerService.

As a bonus, we can also make the following field and method private.

  * InputMethodManagerService#mCaller
  * InputMethodManagerService#executeOrSendMessage()

This is still mechanical refactoring.  There should be no observable
behavior change.

Bug: 192412909
Test: presubmit
Change-Id: I37a4bf39e21955af0e677e576e4eb87d3105c8c6