Expose PngHandler via resources.h.

As a private header for testing purpose. PngHandler additionally loads a
given filename if the one with '/res/images' prefix is not available. It
also provides color_type/bit_depth that are parsed from the PNG file.

This allows reusing the same code for the ResourcesTest (renamed from
ResourceTest).

Test: Run recovery_manual_test on marlin.
Change-Id: I3f939d79a1cb1b83a899847dbe2d51bde15d16d8