This is necessary to examine caller's permission. If caller's uid is not
same as passing package name, SecurityException would be throwed.
This change also clear the identity before calling
BluetoothPan#setBluetoothTethering() in Tethering#setBluetoothTethering.
This is fine because caller already pass permission check before in
ConnectivityService. See the flow below:
ConnectivityManager#startTethering -> ConnectivityService#startTethering
-> Tethering#startTethering -> Tethering#setBluetoothTethering
-> BluetoothPan#setBluetoothTethering
Bug: 134649258
Test: -build, flash, boot
-atest FrameworkNetTests
-manual test with bluetooth OFF/ON
Change-Id: I2140398ad3bbc8076f729c843f0515c654553aaf