Use SystemClock.elapsedRealtime in ImeTracker

Currently ImeTrackerService uses System.currentTimeMillis for the start
and finish time of the requests. However, this could break if the system
wall time is changed (by the user) between a request start and finish.

This also passes the elapsedRealtime value, to be used for the start and
finish timestamps. The currentTimeMillis is kept for convenience in
reading the history in the dumpsys, for the start and last progress
times.

Flag: EXEMPT bugfix
Test: atest ImeTrackerServiceTest
  checking output of adb shell dumpsys input_method
Bug: 342111149
Change-Id: I3e41de1c3df21ba393379b6fd6eab9cd0916ca99