Remove 'platform' signing from test app.

It will instead use the default certificate, and tests will adopt individualized permissions to interact with the service under test.

Bug: 395140628
Test: m -j ; atest
cts/tests/tests/security/src/android/security/cts/IntrusionDetectionEventTest.java
cts/tests/tests/security/src/android/security/cts/IntrusionDetectionEventTransportTest.java
Flag: android.security.internal_log_event_listener
Flag: android.security.afl_api

Change-Id: I44090e927036281fac2369d0f60e28336a529e13