Let ImeTrackerService use our own Handler

This is a preparation before making

  InputMethodManagerService#mImeDrawsImeNavBarRes

per-user instance.

While ImeTrackerService has been using the main thread since it was
originally introduced [1], I guess there was no strong reason to not
use other thread. Let's just use IMMS#mHandler in favor of simplicity.

There should be no semantic change in this CL.

 [1]: I432bab2de58a9df2c421bb00946ab211de445660
      89f36a77

Bug: 345519864
Test: presubmit
Flag: Flag: EXEMPT refactor
Change-Id: Ief87250b8892fefcc645d2324ef5b73e3bab3cb5