audio: Facilitate extension of Module class by vendors

Make interface methods protected so that subclasses can
augment them.

Provide getters for private fields.

Bug: 282568751
Test: atest VtsHalAudioCoreTargetTest
Merged-In: I0e4810f8a4c816c4f673139816e9768f6dc8da7c
Change-Id: I0e4810f8a4c816c4f673139816e9768f6dc8da7c
(cherry picked from commit 57f0dcf7)