Avoid creating multiple death recipients for same observer

Also do a WTF when the same process registers 1000 observers with the
same observer instance.

Fix: 133388082
Test: manual test: Launch leaky app and make sure WTF is raised
Test: manual test: Open contacts app, close, and make sure the
"Total number of unique observers" goes back
Test: manual test: Open contacts app, kill it, and make sure the
"Total number of unique observers" goes back
Test: manual test: Open contacts app while contacts are syncing,
and make sure the activity refreshes itself.
Test: atest cts/tests/tests/content/src/android/content/cts/ContentResolverTest.java
Test: atest FrameworksCoreTests:BinderDeathDispatcherTest
Test: atest FrameworksServicesTests:com.android.server.content.ObserverNodeTest

Change-Id: Id738ca66023c029b92db3d108d99a9fc240fe77c