[remoting android] Generate trimmed-down credits.html.
This copies resources to a central location, so that the Android build
is independent of anything under remoting/webapp.
This provides GN options to tools/licenses.py to only list third-party
projects that are included in the app (currently the "remoting_apk"
target, but followup work will enable us to specify the actual target
for Official builds).
Also stripped out CSS rules that were only meaningful for the webapp.
This also updates tools/licenses.py to use GN from //buildtools to
address a previous breakage on official Android builders.