PackageManager: Fix concurrent access to shared state

Stop multiple threads from all calling add on the same HashMap.

Cleanup the use of a boolean flag by removing concurrent writes
to it.

Change-Id: I8d61fe7294dcb67ee7d0d7de88d90d2051c310e6
Fix Javadoc link for getModifierBehavior()

Change-Id: I571db5147a081fcd7190483ba9e7a45b46db224d