Clear caller identity while checking Tether Change permission

When changing the Bluetooth tether settings, the call comes from the
system UID but the app op package name of the Bluetooth app is used
and this causes a mismatch between UID and package. This commit
temporarily clear the caller identity while doing the Tether Change
permission check.

Bug: b/37746850
Test: Manual through the Settings app on device
Change-Id: I4835f940dea28f183fbddcc467723a9e03e759fd