Update Android SDK and Android System SDK to R.

Switch to the R SDK and system SDK, and add the WebView code to support
WebView R APIs.

Bug: 1106471
Change-Id: I4fa349ff6b79983730bca609b95532b019e3e05d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310800
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Anna Malova <amalova@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792225}
GitOrigin-RevId: 63a24caef73eec3e24845bbcc3fecbb70f18507f