android: Remove rule_trigger targets.

The rule_trigger target doesn't serve any useful purpose since all the
output files from the rule are added to extra_outputs, which is already
depended on by the final target. It causes problems when trying to do a
multiarch build since the same rule trigger target name is calculated
for both architectures, so remove it.

BUG=chromium:358141
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/222043002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1885 78cadc50-ecff-11dd-a971-7dbc132099af
android: Allow disabling the generation of alias targets.

The android backend mangles the names of targets in its output in order
to ensure they don't clash with each other or with existing Android
build system targets, since they are required to be unique. It also
generates aliases referring to their original names as given in the gyp
input, for usability and test purposes, but this can cause namespace
issues. Add a generator option allowing the generation of these aliases
to be disabled, which will also disable generation of the
"gyp_all_modules".

BUG=gyp:341
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/222243004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1886 78cadc50-ecff-11dd-a971-7dbc132099af