Avoid IME restart for configChanges

Handle onConfigurationChanged() in order to prevent restarting
InputMethodService everytime. We introduce a new API attribute
"configChanges" in InputMethod(attrs.xml) which when declared
by IME, will be responsible for handling mentioned
configuration changes.

This CL re-introduces [1] with fix: Use new Configuration instance for
IMS#mLastKnownConfig and also handle followup comments.

[1] Ib94fddadb0dae648cf73a4c1642e51edebd19f50

Note: this change has no impact for devices not using DisplayAreas.

Bug: 167948419
Test: atest InputMethodServiceTest
    Manually:
      1. Patch Ie91e7a8e06b80864ef9409031e8543858552d70d to use dual
         display area.
      2. Open applications with editors on both display areas.
      3. Attach a debug point for IMS#onConfigurationChanged().
      4. Make sure IMS#resetStateForNewConfiguration() is not called
         when IME moves between these two identical DisplayAreas
    Also verify that bug 182604598 don't happen.

Change-Id: I43b6b80cdb35410554412ee1d3b0917ee3198272