BroadcastQueue: post finishReceiver() as async.

In the typical case, the remote process calls back around to
finishReceiver() once it's finished.  In cases were we shortcutted
delivery, we had been calling finishReceiver() directly, but that
results in holding the AMS lock for a long period of time as we
recursively deliver the next pending broadcast.

This change adjusts that shortcutting behavior to post through a
message, so that we release the AMS lock, giving other pending
operations a chance to run.

Bug: 255936508
Test: ./frameworks/base/libs/hwui/tests/scripts/prep_generic.sh little && atest ActivityManagerPerfTests:BroadcastPerfTest
Change-Id: Ic7094df0a1d2aa1e2ac1e22c1fd711c245d8eb86