Refactor DynamicIndexableContentMonitor

Refactor content monitoring code into a few singletons to keep alive
while Settings app is running.

Bug: 32995210
Test: Manually installing/uninstalling AOSP LatinIME.apk while
      Settings app is/isn't running, then search AOSP.
Test: Connecting/Disconnecting Anker bluetooth keyboard while Settings
      app is/isn't running, then search Anker.
Test: Added Robolectric test for DynamicIndexableContentMonitor.
Change-Id: I588e33be169fc9677d41c3daa59ab400f04f6419