android buildbot: Default to ninja's default target instead of All.

Before https://codereview.chromium.org/149103002/, chrome/android's default
target was 'All', so no behavior change. After that lands, the default target
is 'all', so the bots will build a few more targets (17 translation units more,
at the moment).

Together with the change linked above, this might enable the removal of
|android_app_target|.

BUG=none
R=yfriedman@chromium.org

Review URL: https://codereview.chromium.org/149213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247607 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247607