SyntheticPasswordManager: consolidate hex encoding logic

Remove the in-class implementation of hex encoding and just use
HexEncoding, which was already being used in one place.

No change in behavior since the encoding remains the same.  In
particular, upper case A-F is still used instead of lower case a-f.

Test: atest SyntheticPasswordTests
Change-Id: I493e4b894f76670bcb9cba8a8a774f3f5832e769