ChunkedMacVerification does non constant time comparison on Mac

[tholenst]

In tink-java version 1.21.0 and all versions which were released before that, ChunkedMacVerification did not do constant time verification.

This was finally fixed on June 8 in 175df91.

This issue serves as a reference for this problem. Note that an issue was filed on June 11 reporting this (175df91) but since it was already fixed I closed the issue. Nevertheless, now I need a reference issue, so I open a new one.

[LuisCastellanos-dev]

I've opened PR #77 which addresses this by replacing Arrays.equals()
with MessageDigest.isEqual() in LegacyFullMac.verifyMac().
Please let me know if the approach looks correct.

[tholenst]

Sorry for the confusion, this is already fixed. Your change #77 is not needed.