AudioService: Fix STREAM_MUSIC volume persistence

across reboot

Currently, the media (STREAM_MUSIC) volume index
is stored under current user but read back form
system user after reboot, causing volume not
persistence after reboot.

This change update the storage logic to persist STREAM_MUSIC
volume under UserHandle.USER_SYSTEM, ensuring alignment between read and
write paths and reliable volume persistence across reboots.

For reference – commit that preserved music stream volume from one user to the next:
- https://android-review.googlesource.com/c/platform/frameworks/base/+/1893662



Test: -atest CtsMediaAudioTestCases
Flag: EXEMPT bug fix

Signed-off-by: Piyush Raj <piyushrj@amazon.com>
Merged-In: I422f3c44460d23ffe9a4945b1ebaf3b17b53dccd
Change-Id: I422f3c44460d23ffe9a4945b1ebaf3b17b53dccd