Change external capture state notification mechanism

Previously, external capture state notifications were delivered from
the audio policy manager to the sound trigger service as normal
"forward calls". However this creates a cyclic dependency between
sound trigger and APM, which is especially problematic during startup,
when the audio server is assumed to start prior to the system server
and thus cannot block on the sound trigger service becoming available.

With this change, the sound trigger service would register a callback
with APM for these notifications.

A small refactoring was needed in order to keep the internal interface
of sound trigger classes having the setExternalCapture() methods,
while removing them from the external interface.

Test: Manual verification, checking the logs, validating behavior in
      response to killing the audioserver process.
Bug: 146157104
Merged-In: I4b4467fbac3607ee170394dc1b3309e7e3d422d8
Change-Id: Ibf68a5e5681afca9c5b1b821bb8ac18a2eebb4ca