Identity: Only run Identity Credential CTS tests if HAL is implemented.

Now that the Identity Credential HAL is optional, use assumeTrue() to
only run tests if the HAL is actually implemented. Previously we ran
the tests only on devices launching with Android 11 (by inspecting
ro.product.first_api_level).

Also do this check for attestationTest which previously was run on any
device causing a lot of failures.

Bug: 151660241
Test: atest android.security.identity.cts
Merged-In: I52a699e8ffd00773656240f129c6b261e7b51db3
Change-Id: I602b292aaf16d09eb4d32a08755188ac76066ddc