identity: Fix uncompressed form encoding of P-256 EC Public Key.

Properly encode an P-256 EC Public Key in uncompressed form, in
particular ensure that the resulting blob is always 65 bytes long as
is expected.

Was able to reproduce this with about 4% failures running a
test. After the fix didn't get a failure in 1,000 runs.

Also remove unused Util.integerCollectionToArray() function.

Bug: 239857653
Test: atest --rerun-until-failure 1000  android.security.identity.cts.ProvisioningTest#testProvisionAndRetrieveMultipleTime
Change-Id: I9a8a5570fde5a80f74632606126cdfcc1f6c7c99