Don't hold lock when calling externally

Wound up in a deadlock. We held mPackages lock and called the mount
service. This called into the AppOps service which tried to obtain
its lock. On another thread, we held the AppOps service lock which
called into PackageManager service and tried to take mPackages.

Ideally, we'd never hold an internal lock when calling externally.
In practice, that's not always fixable [for example, code change
would be too great to not hold the AppOps lock when calling into
the PackageManager service].

Bug: 28169075
Change-Id: If0a7f1361c7be9385b1f629f68ebc0556b08ab06