Start proguarding google play services.

This is necessary to avoid the dex limit after upstreaming.

BUG=484934

Change-Id: I7047a356f53c2fecb143e4e336eb29db5e5f590b
Reviewed-on: https://chromium-review.googlesource.com/272306
Reviewed-by: Chris Hopman <cjhopman@chromium.org>
Tested-by: Aurimas Liutikas <aurimas@chromium.org>
Tested-by: Newton Allen <newt@chromium.org>