[aapt2] Improve dump chunks for string pools, fix diff

- `dump chunks` command now prints extra header data for the
  string pool, and outputs all style information for styled
  strings.

- `diff` command used to only correctly compare the first span
  in the StyledString data type, making them appear different
  if there was a string with more than one span

Flag: EXEMPT bugfix
Test: atest aapt2_tests
Change-Id: I377718c03d6a464cb4db22399b0f067e6a6e04d6