hasRestrictedModeAccess: Clear calling identity when checking network capabilities

* This code ends up being called from Datura, the firewall app,
  which would fail with:
java.lang.SecurityException: Package android does not belong to 10139
   at android.net.NetworkPolicyManager.removeUidPolicy(NetworkPolicyManager.java:352)
   at org.calyxos.datura.settings.SettingsManager.setAppRestriction(SettingsManager.java:108)
Caused by: android.os.RemoteException: Remote stack trace:
   at android.app.AppOpsManager.checkPackage(AppOpsManager.java:8552)
   at com.android.server.ConnectivityService.getNetworkCapabilities(ConnectivityService.java:2042)
   at android.net.ConnectivityManager.getNetworkCapabilities(ConnectivityManager.java:1563)
   at com.android.server.net.NetworkPolicyManagerService.hasRestrictedModeAccess(NetworkPolicyManagerService.java:4133)
   at com.android.server.net.NetworkPolicyManagerService.getNewRestrictedModeUidRule(NetworkPolicyManagerService.java:4116)
Package android does not belong to 10139
  where 10139 is the uid of the firewall app
* This should be safe since we're only checking the capabilities here,
  and not actually returning anything anywhere - just a boolean

Change-Id: Ia0b5790be19ad5aece0d30ae8e1a465f45f4fed6