Replace mHandler.sendMessage() with sendToTarget()

This CL mechanically replace

  mHandler.sendMessage(msg)

with

  msg.sendToTarget().

This should be safe because we always use obtain Message objects from
the same Handler object in InputMethodManagerService.

There should be no observable behavior change.

Bug: 192412909
Test: presubmit
Change-Id: I9fa657cfff18c4111e0fad4d4e354c011c368772