Allow "." as a valid internationalised domain name.

Rejecting "." causes issues for developers using DNSSEC libraries
(see bug) and is inconsistent with RI.

My upstream fix to ICU was initially rejected so adding a workaround in
java.net.IDN until I can resolve that.

Bug: 113787610
Test: atest CtsLibcoreTestCases:org.apache.harmony.tests.java.net.IDNTest
Change-Id: Ic10099eb106980389dc19edf0e20c7105377ff6e