Expose public interface to starting and stopping IME tracing

The added interface makes it possible to start or stop IME tracing from
another process. This will be used together with the DevelopmentTiles to
ghather IME tracing data when the WinscopeTrace tile is used.

Bug: 154348613
Test: flash a device together with the other change in this topic
      enable the WinscopeTrace tile and do some actions
      disable the WinscopeTrace tile
      do a bugreport and visualise contents in Winscope
      or just check contents of /data/misc/wmtrace/ through adb
Change-Id: Id03ede35f71c3d513dea50ef69e4ed4ce05a8d53