Remove IMMS#MSG_HIDE_SOFT_INPUT

This CL inlines

  InputMethodManagerService#MSG_HIDE_SOFT_INPUT

handler as it has always been executed from the calling thread.

There should be no observable behavior change.

Bug: 215609403
Test: presubmit
Change-Id: I1015c01b366d50f7faa885d7e92384cc1bed7e2e