Unregister all the receivers during test teardown.

Bug: 253696106
Test: atest services/tests/servicestests/src/com/android/server/net/NetworkPolicyManagerServiceTest.java
Change-Id: I15eb40a92790373182e79bdd15bf01d66520d1bf
Merged-In: I15eb40a92790373182e79bdd15bf01d66520d1bf
(cherry picked from commit 31b33be1d9a4fb82944e5cd8f92a963b08dbe447)
1 file changed