Change allow button to a downward button to scroll the permission list

1. If the user hasn't scrolled to the bottom of the permission list, display a button with a downward arrow.
2. Each tap of the downward button scrolls the permission list to reveal the next permission.
3. Once the user has scrolled to the very bottom of the permission list, the "Allow" button becomes visible.

Fix: 391252796
Test: cts + manually
Flag: EXEMPT bugfix
Change-Id: Iac25d681edaf3604230c6d2f7ac8133a8a355ca2