Android: use a public URL for system SDK

This uses a public URL for the "URL:" field in the README. Apparently
this is the URL which is shown by chrome://credits/ for Chrome on all
platforms, so it's not helpful
to link to "go/ab."

This expands the README text to point to the internal guidelines for
updating this dependency, which should already be more helpful than
pointing at go/ab directly.

Bug: 1045626
Test: tools/licenses.py scan | grep 'android_system_sdk'
Test: build chrome, navigate to chrome://credits, click "homepage" link
Change-Id: Id2e34ffb52dbe8e0f64c3f50ea653f9133a35871
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036969
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Auto-Submit: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738257}

third_party/android_system_sdk/README.chromium

 Name: Android System SDK
-URL: http://go/ab
+URL: https://android.googlesource.com/platform/frameworks/base
 Short Name:  Android System SDK
 Version: 0
 Revision: QPP6.190730.006
@@ -11,5 +11,8 @@ Description:
 System SDK stubs for compiling Android Webview and Monochrome targets.
 Revision is the BUILD ID of the Android release build.
 
+Because prebuilt system SDKs are not distributed publicly, Googlers will need to
+follow http://go/clank-webview/next-layers to update this.
+
 Local Modifications:
 None