GN: add skia_android_serial and push_$foo targets when it's set

E.g.
    $ ninja -C out push_dm

By default (ninja -C out) everything is built and pushed if skia_android_serial
is set.  Dependencies are tracked, so incremental builds push only executables
that have changed.

Change-Id: I586d81791f5f877b173cf61ed68fa9aab96234d2
Reviewed-on: https://skia-review.googlesource.com/6616
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed