Apply delivery group policies to ACTION_DROPBOX_ENTRY_ADDED.

- Policies are applied to only broadcasts corresponding to
entries with low priority tags.
- The "set-defer-until-active" policy is applied so that the
broadcast targeted to apps in the Cached state is deferred until
they come out of that state.
- The "deliver-merged" policy is applied so that if there are
already pending broadcasts waiting to be delivered when a new
broadcast is sent, they will be merged into one.

Bug: 268560540
Test: atest ./services/tests/servicestests/src/com/android/server/DropBoxTest.java
Test: atest ./core/tests/coretests/src/android/os/BundleMergerTest.java
Test: atest ./services/tests/mockingservicestests/src/com/android/server/am/BroadcastQueueModernImplTest.java
Test: atest ./tests/DropBoxManager/src/android/dropboxmanager/cts/DropBoxTests.java
Change-Id: I805380fc0e762be2d34ce244bfadb593a980af1f