[aapt2] Improve "dump chunks" output

- print the sparse/dense string for each type chunk

- skip looking for a non-string ID in the string pool

- print the bag attribute's name-id in hex and on the same line

Test: aapt2 dump chunks on a few apks
Flag: EXEMPT bugfix in a tool
Change-Id: Ib1c55fe6e5577a84cd4fc5682940e5bfbb3031fa