audio policy: different default config for system volumes

When a volume curve is not found for a volume group
associated to a system strategy, use AUDIO_STREAM_PATCH
instead of AUDIO_STREAM_MUSIC as default config.

Bug:148588565
Test: play music and check volume
Change-Id: Ib3fb2a1b7b5f3118bf16a3cc9d8e578fa0431645
Merged-In: Ib3fb2a1b7b5f3118bf16a3cc9d8e578fa0431645