AAPT2: Iterate over UTF-8 string by codepoints

Iterating over a UTF-8 string by codepoints ensures that
unicode characters do not get sliced. Otherwise the resulting
string could contain malformed characters.

Bug: 62839202
Test: make aapt2_tests
Change-Id: Ia0c44fbceb7dcfa11e77a1a77011da0f5466e342
7 files changed