[aapt2] Add a way to set command line from environment

Sometimes one needs to test or alter aapt2 behavior inside the
system build. This CL allows for a simple way of adding command
line flags when you're not the one running it directly:

_AAPT2_FLAG_NAME adds the '--flag-name' command line parameter
before all actual parameters passed through the command line,
while AAPT2_FLAG_NAME_ adds it after all of those. This way
it's easy to either add a default value, or a hard override to
whatever you need.

+ Allow '-h' shorthands for the long '--help' switches when it's
  unambiguous

Test: build + boot
Flag: EXEMPT command line tool
Change-Id: I3effd25ec207d671ba87200b530601525382240d