Update supervision robolectric activity tests to rely more on shadows.

This will allow us to exercise more of the real Activity lifecycle
rather than simply verifying expected behaviors by spying on the
Activity object.

Bug: 403575468
Flag: TEST_ONLY
Test: atest ConfirmSupervisionCredentialsActivityTest
Test: atest SupervisionCredentialProxyActivityTest
Change-Id: Ie8838e046dd1c81dcfad9132fde71fe5e1880da6