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