Clean up leaked screenshot connections

If a subsequent screenshot is taken after we bind to the screenshot
service, but before we get the onServiceConnected callback, we
leak the connection to the service, preventing it from ever getting
shut down. Fix this by unbinding the old connection before connecting
with the new one.

Bug: 242862442
Fix: 242862442
Test: forced DisplayPolicy to take two consecutive screenshots

Change-Id: Iffb62170266a2c1bb332bfb416f0f5e19c5236fb