Verify Pattern, step 2: Integrate behavior change for Pattern.split().

This CL integrates a behavior change that was forgotten in the update
from OpenJDK 7u40 to 8u121-b13.

This CL also adds a test to guard against regressions.

Bug: 109659282
Test: testMatchBeginningOfInputSequence() fails before this CL, but
      passes after this CL.
Test: make cts && cts-tradefed run cts -m CtsLibcoreTestCases

Change-Id: Ie7340629a0954fa231d1a1f2a51885c0774ed436