Only deliver latest WIFI_DISPLAY_STATUS_CHANGED.

We've heard reports of apps getting confused about hearing thrashing
WIFI_DISPLAY_STATUS_CHANGED broadcasts when they're thawed.  To
mitigate this, only deliver the latest instance of the broadcast,
replacing any stale broadcasts.

Bug: 273923757
Test: manual
Change-Id: Ie8e4a7213f7b4838b33fdc6cf4ccca9520ad49f9