Fix BroadcastReceiver leakage

This CL ensures that a shared PackageMonitor is unregistered when a
settings activity pauses.

Bug: 34541282
Test: Manually installing/uninstalling AOSP LatinIME.apk while
      Settings app is/isn't running, then search AOSP. Verified that
      intent receiver isn't leaked from Settings app.
Test: Add assertion to Robolectric test to ensure intent receiver
      isn't leaked. Build RunSettingsRoboTests and verify all passed.
Change-Id: I5bacdfe478eaa750c61341c14e79eb2cd3fdef3a