Switch Base64 decoder implementation used in harmony SSL tests

These tests were using libcore.io.Base64.decode() for test data. Since
that class is deprecated and the tests weren't depending on the subtle
behavioral differences relative java.util.Base64.getDecoder().decode(),
this CL converts them to the latter implementation.

Bug: 31292683
Test: cts-tradefed run cts -m CtsLibcoreTestCases -a arm64-v8a

Change-Id: I635625ceae9d6a33a4909101e65363d7052263d9