locksettings: use ArrayUtils.concat(byte[]...)

Use the new byte array concatenation utility method where appropriate.

No change in behavior intended; this is just a cleanup.  The new method
accepts null arrays, whereas most of the previous workarounds didn't,
but I don't see anywhere where this would make a difference.

Also use HexEncoding.encodeToString() in a couple places.

Test: atest com.android.server.locksettings
Test: atest LockscreenCredentialTest
Change-Id: I203018d7cba2721a9d0c4988708df7147e396dbe