audio: Add IStreamCommon.prepareToClose method

This method is needed to implement HAL modules that
are proxies for other subsystems, for example
the "bluetooth" and "r_submix" modules. This method
replaces string parameters "exiting=1" and "closing=true"
which the framework sends to streams prior to closing
them in order to unblock the I/O thread of the stream.

Bug: 270731693
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I3d13fe34535ab853c9f8237a08c31cdafadbb390