Add public Android S SDK to third_party/android_sdk

Also add the source for Android R.

Bug: 1223110
Change-Id: I0a4ae2675c4873532298dc8a9e426131099e0930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3027663
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#901616}
NOKEYCHECK=True
GitOrigin-RevId: 486d78681885f92a85e1aff9043f75c7e4a98de4