APM: Provide mix port flags via 'listAudioPorts'

Flags are now provided for mix ports via the 'active_config.flags'
field of 'audio_port_v7'.

To enable this, the following changes were made:

1. Use 'AUDIO_PORT_CONFIG_ALL' config for the export in
   Audio{Input|Output}Descriptor::toAudioPortConfig.
2. Fix duplication of 'mFlags' field between AudioOutputDescriptor
   and its base class AudioPortConfig.
3. Add 'mFlags' field to AudioInputDescriptor which is a reference
   to the relevant part of AudioPortConfig.mFlags (same mechanism
   as in 2.)
4. Verify behavior in
   AudioPolicyManagerTestWithConfigurationFile#ListAudioPortsHasFlags

Also, 'test_audio_policy_configuration.xml' was updated to V7 syntax
to exercise more paths in APM.

Bug: 218507839
Test: atest audiopolicy_tests
Change-Id: I78fbd53dde15a90eca104aa17c68282563f8633a