Fix storage directory API docs

Android 10 did not support file path operations for apps in
scoped storage. But Android 11 and above do support file path
operations for such apps.

Fix docs for getExternalStorageDirectory() and
getExternalStoragePublicDirectory() to reflect the current level of support.

BUG: 153562415
Test: m docs

Change-Id: I8bc2a4aa266dbbcc391b415e6209ed6e5d56fe97