In crrev.com/c/1866080, catapult and android_ndk were added for
supporting standalone Android builds, which increased ANGLE's fresh
checkout time and disk occupation a lot. For ANGLE embedders, this can
be mitigated by putting them behind a condition, where checkout_android
is a builtin variable controlled by target_os in the .gclient file. For
standalone builds on Linux, Android will be added to target_os by
default for compatibility with previous behavior.

Bug: None
Change-Id: I0b258c6fc7f62ca65e705b8c31c9ace5dffd4030
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2216709Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiajie Hu <jiajie.hu@intel.com>