hal: Fix incorrect FM volume after reboot

Reboot the phone, start FM playback and it plays back at
max volume regardless of previous Media volume.
The audio HAL is caching the volume that is being sent to
driver after conversion.
Fix the issue by caching the value set by the policy manager.

Change-Id: I2c416fc72d354b368890afcdd51b83e662b0ce6e
CRs-Fixed: 579377