Enable full RIOTest.testZeroLengthPrefix test on all kernel versions

Also add new tests to guarantee that RIOs are being accepted as well as
the rejection power of min_plen.

Bug: 33333670
Test: net_test passes on android 3.10, 3.18, 4.1, 4.4, 4.9 common and
device variants.

Change-Id: Ib7c0adff7f081e36d279ff88c5133adb6740a81b