Clear caller identity before updating restrictions

Fix Android 15 regression in which apps with MANAGE_NETWORK_POLICY are
not able to manipulate policies unless they are also AID_SYSTEM.

Bug: b/371004144
Issue: calyxos#2733
Issue: calyxos#2701
Test: Manual: Install test APK from linked bug to userdebug build.
Try changing toggles. Should not crash.
Change-Id: I0536ec7ab412ea6102092d44c0c938a986cf2c9d