Have getVolumeList() take a userId instead of a UID.

getVolumeList() previously accepted a uid and a packagename. This
approach has a few issues:
1) The uid wasn't actually checked in the implementation to match the
calling UID, nor was the package name checked against the UID in all
cases.
2) The implementation assumed that the UID also specificies the userId
that we want to get the volumes for. This was historically true, but
with the new app cloning implementation, MediaProvider running in user 0
might ask for volumes in the clone user.

To fix this, explicitly pass in the userId that we're requesting volumes
for, and verify that the calling UID and the passed in package name
match.

Bug: 206624903
Test: atest packages/providers/MediaProvider
Change-Id: Idffbb4ae7740c1b9725af0974948dc55bf1f1fde