Any Android specific modifications to upstream libpng (the version
specified in METADATA) should be listed here:

(1) Android has added the following files.
    pngusr.h
    Android.bp
    CleanSpec.mk
    MODULE_LICENSE_BSD_LIKE
    METADATA
    OWNERS

(2) pnglibconf.h
    This is copied from scripts/pnglibconf.h.prebuilt.
    Android has undefined PNG_WARNINGS_SUPPORTED and
    PNG_READ_OPT_PLTE_SUPPORTED.