Fix bug where aapt2 optimize obfuscated color-* resources

aapt2 optimize --enable-resource-path-shortening should ignore res/color
resources since android uses the path while loading them. This cl fixes
a bug where res/color-* (eg: res/color-v21) resource paths are shortened
when they should not be.

Test: make aapt2_tests
Bug: b/75965637
Change-Id: Ibbc3fe4d2e6bd4f31765a5aa85980907dc013ecd