Limit broadcast rate for low priority DropBox entries

DropBox entry spamming can negatively impact system and apps listening
to the ACTION_DROPBOX_ENTRY_ADDED broadcast.
Global settings with the DropBox tag prefix can now mark low priority
tags. Low priority tagged entries will have their
ACTION_DROPBOX_ENTRY_ADDED broadcast delayed and the broad cast will be
dropped in spammy situations.

Bug: 119132031
Test: atest CtsDropBoxManagerTestCases

Change-Id: I56554a15e0afb6e1686a33c59b3d6a8d426c2fc6