Remove IMMS#MSG_START_INPUT

This CL inlines

  InputMethodManagerService#MSG_START_INPUT

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

This CL also fixes a comment bug about IMMS#mEnabledSession.

There should be no observable behavior change.

Bug: 215609403
Test: presubmit
Change-Id: I402e93be660aaabcef64b2cb474fc3985d749a40