audio_policy: modify few methods to appropriately override base

Few methods in AudioPolicyManagerCustom is not overriding its
base implemenation due to signature mismatch. Since they act
as separate method they are not getting called to override
base implemenation.
Correct signatures to implement overriding instead of overloading.
Remove custom stopOutput implementation since it is not adding
any change w.r.t. base implementation.

Change-Id: I68816eeb95342b574b34a76df9c6ad110f14823d