Android backend: fix references to source top dir.

$(gyp_intermediate_dir) is sometimes an absolute path, and so trying to
convert it from relative to absolute by prepending
GYP_ABS_ANDROID_TOP_DIR is wrong. Use make's $(abspath) instead to
absoluteify the path. This means we don't need GYP_ABS_ANDROID_TOP_DIR
any more, so remove it entirely.

BUG=
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/16087003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1636 78cadc50-ecff-11dd-a971-7dbc132099af