Handle SCREEN_ON/OFF broadcasts without blocking.

NetworkPolicy currently uses a single background thread to process
various broadcasts.  When processing other broadcasts, this thread
can block our handling of SCREEN_ON/OFF, which are sent as ordered
broadcasts.

This change moves SCREEN_ON/OFF handling to the main thread, and
dispatches a one-way message to the background thread, allowing the
ordered broadcast to always proceed.

Bug: 6677047
Change-Id: I52de2c7b75beb8059bb87e123689ba4a9c4ae349