Remove IMMS#MSG_SHOW_IM_SUBTYPE_ENABLER

This CL inlines

  InputMethodManagerService#MSG_SHOW_IM_SUBTYPE_ENABLER

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

There should be no observable behavior change, except for the case

  InputMethodManagerService#getCurMethodLocked()

was returning null and the client process was receiving NPE, which
should no longer happen.

Bug: 215609403
Test: presubmit
Change-Id: I868af935ffdc794669c5cb5d0437c919533ac2ab