Add audio level monitoring capabilities in Visualizer effect

Extend the visualizer audio effect with the capability to query
 peak and RMS values for the currently playing audio.
Values are expressed in mB and are retrieved as an array of
 int values in the native layer, and written directly as
 object fields for the JNI.

Bug 8413913

Change-Id: I808075a18e61f85c566544a2bdaae10e5c4a644b