Avoid a deadlock between PMS and AppOpsService

Don't check for external storage access rights from MountService
for system server. Otherwise there's a case where AppOpsService
is locked and PackageManagerService calls into AppOps with its
own lock held and is unable to do an AppOps check via this path.

Bug: 22522725
Change-Id: Ib4cf914638905de391384aa5122e691c5a7140ec