Updates to AudioPolicyManager refactor

Use protected access in AudioPolicyManager class for
 - engine field
 - querying phone state
 - isStrategyActive

Fix inclusion of AudioPolicyManagerInterface header.

Use "APM_" prefix in audio policy conf parsing code to avoid
 naming conflicts with some HALs.

DeviceDescriptor and its collection class are good friends now.

Change-Id: I236d62a2b8a4b6bf68dd515932650b912577c145