Passing caller package name to setBluetoothTethering

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