audioservice: fix media volume reset at boot

mSafeUsbMediaVolumeIndex must be initialized after createStreamStates()
because it relies on audio policy having correct ranges for volum indexes.

Also change the safe volume threshold for USB headset from -15dBFs to
-30dBFs to account for max EQ gain.

Bug: 63462140
Bug: 63388412
Test: Verify USB headset media volume is unchanged after reboot
when no SIM is present.

Change-Id: I5252d97e3f3dcde7568681abb066752ef0d936d2