audio policy: fix STRATEGY_PHONE routing when not in call

commit 2517af3e unintentionaly mapped STRATEGY_PHONE to
STRATEGY_MEDIA when not in call. This mapping is only for
STRATEGY_DTMF.

Also group filtering of available output devices according to
strategy in a helper method and apply the filter also before
checking for preferred devices for a given strategy.

Bug: 179642678
Test: atest AudioCommunicationDeviceTest
Change-Id: Ia47b81ee6aff9a49a83f40870eb811b493141f5b