Add flags to BroadcastDispatcher

Starting soon, one of Context#RECEIVER_NOT_EXPORTED or
Context#RECEIVER_EXPORTED is required when registering broadcasts. This
CL adds an optional argument to BroadcastDispatcher#registerReceiver to
add flags. The default is RECEIVER_EXPORTED, as it's the backwards
compatible behavior.

Note that many actions listened from SystemUI are platform actions and
as such require RECEIVER_EXPORTED.

This CL does not address receivers registered directly with Context.

Test: logs and dumps
Test: atest SystemUITests
Fixes: 198424247
Change-Id: I80d0cee9347c22c4246bd3da8abc9b040d35490a