Remove IMMS#MSG_SHOW_SOFT_INPUT

This CL inlines

  InputMethodManagerService#MSG_SHOW_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: I122ade0fc2b343d46e7e8a4f3a02403d3c349533