Communicate flags from systemui async

Flags are no longer statically compiled into the shared library.

A "GET_FLAGS" api has been added to SystemUI that returns a list
of Flag objects, as defined in SystemUI.

Communication happens via a simple "ordered" broadcast.

FlagManager#getFlagsFuture() returns a ListenableFuture pointing at
the list of flags. Be sure not to call Future#get on the main
thread - the call will block indefinitely as the broadcast receiver
is handled on the main thread.

Instead, add a listener to the future or call #get on a separate
thread.

Bug: 203548827
Test: manual
Change-Id: I8720e0905662a6371e7aabf04341df46b8943a9c