Remove unaudited flag from SafetyController receiver

The package change receiver in SafetyController was updated with an
unaudited exported flag to ensure the receiver meets the new
requirement that apps targeting U+ and registering a receiver for
non-system broadcasts must have a flag indicating the exported state
of the receiver. This receiver may have been incorrectly reported
as requiring a flag due to it including a data scheme; since this
receiver is registering for the system broadcast PACKAGE_CHANGED,
a flag is not required.

Fixes: 257410439
Test: atest SystemUITests:com.android.systemui.statusbar.policy.\
  SafetyControllerTest#addingFirstListenerRegistersReceiver
Change-Id: Ia63eed1761aec7b0b7a4781f9ad6d6e995a625a2