Fix GraphemeBreak test failures.

GraphemeBreak.tailoring/GraphemeBreak.genderBalancedEmoji start failing
after ICU update to 58. The failure is around Rule GB9 in Unicode Standard
Annex #29. GB9 forbids breaks before extending characters and before ZWJ.
However the implementation in minikin only checks for extending characters.
It used to work with Unicode 8.0 since ZWJ had the Grapheme_Cluster_Break
property of Extend in Unicode 8.0 but it no longer has that property in
Uniocde 9.0.
Thus, we need to check for ZWJ explicitly.

At the same time, this removes manually added PREPEND characters case from
tailoredGraphemeClusterBreak which is already supported in ICU 58.

Test: minikin_tests passes
Bug: 34117643
Change-Id: Ib46d48bebe4a866208e050d7defc715c61fcbeb1

• libs/minikin/GraphemeBreak.cpp[diff]