Support android as a target platform.

Provide an is_android condition, and set is_posix to true when the
target platform is android to match the OS_* defines in Chromium.
grit_info didn't previously support -t to set the target platform, so
add that as well.

BUG=chromium:239434

Patch by torne@chromium.org, original issue 14813026

Review URL: https://codereview.chromium.org/15051003

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@120 7262f16d-afe8-6277-6482-052fa10e57b1