Clear binder identity before calling PackageManager API

With the introduction of app enumeration restriction in R,
PackageManager will restrict package visiblity if the caller
is not privileged. This caused regression in existing system
server code where binder identity is not cleared prior to
making PackageManager calls.

Bug: 150398249
Test: com.android.cts.devicepolicy.MixedDeviceOwnerTest#testAlwaysOnVpn
Change-Id: I611eb5768bfb73f01c63e6ab02d90f1178f8ec37