Implement --xml-dir option to android2grd.
Implement --xml-dir option to android2grd.

The --xml-dir option to android2grd was previously a no-op. This
implements it and clarifies the meaning of several other options.

BUG=none

Review URL: https://codereview.chromium.org/11663002
skidzen/grit-i18n,willbittner/grit-i18n,invalidstatement/grit-i18n,joisig/grit-i18n,sevansahumIlovemuhammad/grit-i18n,CrankWheel/grit-i18n,qjw/grit-i18n,korusdipl/grit-i18n,metasmile/grit-i18n,metasmile/grit-i18n,sevansahumIlovemuhammad/grit-i18n,megamanfx/grit-i18n,skidzen/grit-i18n,lhopps/grit-i18n,korusdipl/grit-i18n,lhopps/grit-i18n,invalidstatement/grit-i18n,willbittner/grit-i18n,qjw/grit-i18n,joisig/grit-i18n,megamanfx/grit-i18n