Fix isMountDisallowed to check permissions on the correct user

Currently isMountDisallowed checks permission on a calling user, which
is always the system user, since the message handler runs on the
background thread.
This is not a problem on non-HSUM devices, since DO provisioning
happens on the system user and in COPE the permissions are set on the
parent profile, which is also the system user.
On HSUM devices this is a bug, since DO provisioning happens on user10
and COPE's parent profile is user10 as well.

Check permissions on the user which volume is mounted to instead.

Test: Verified both on HSUM and non-HSUM devices that the policy WAI.
Test: atest StorageManagerServiceTest
Flag: EXEMPT bugfix
Bug: b/346334511
Bug: b/403258207
Change-Id: I014e88bf2797c69d5546bcd02fe8f0933a11ff0e