Accessibility fixes for clipboard overlay

Add/fix content descriptions for content preview, share button and remote copy.
Add labels for the overlay window and the edit activity.
Reset timeout on hover so that a11y exploration keeps the UI up.

Bug: 232485623
Bug: 232494683
Bug: 232487324

Test: Manual evaluation with talkback enabled.
Change-Id: I995083a8b00f68d85d0a1c3f699a7f27a3c40aad