Use structure aware fuzzing for pngs

Bug: 142252770
Bug: 169137236
Bug: 169139756
Test: this

Features built into the PNG format slow down fuzzing (e.g. CRC, encoded
lengths) [1]. Use structure aware fuzzing for PNG to do a better search
through the input space. Add png_mutator.h (unchanged from [2] under the
Apache 2.0 license). Split imagedecoder_fuzzer into two targets: one for
PNGs (since the mutator converts all inputs into PNG), and one for the
other formats.

Move fuzz_imagedecoder.cpp and the corpus/ into the new fuzz/ directory,
to keep fuzzing separate from the libjnigraphics library. Remove
png_test.png from the corpus; structure aware fuzzing will do a better
job of fuzzing PNGs, and the generic one should focus on other formats.

[1] https://github.com/google/fuzzing/blob/master/docs/structure-aware-fuzzing.md
[2] https://github.com/google/fuzzer-test-suite/blob/51356066dc70c43c9da0ad98e887684a0394860f/libpng-1.2.56/png_mutator.h#L1

Change-Id: I8aebb0d0abfa18793334f2d217b28a51d096123a