Fix the SYSTEM_UID check in ConfirmSupervisionCredentialsActivity.

Previously it was directly comparing the uid instead of extracting the app id, which doesn't work outside of user 0 (isn't multi-user aware).

Bug: 392961554
Flag: android.app.supervision.flags.enable_supervision_settings_screen
Test: atest ConfirmSupervisionCredentialsActivityTest
Change-Id: If9fe1073785406a5140def81a161816ce051ddbb