Rename attrs/files shared between clipboard and screenshots

The clipboard overlay reuses much of the same UI elements that the
screenshot UI does. Rename these from e.g. "screenshot_X" to
"overlay_X" to reflect the shared usage.

Bug: 195554988
Test: manual
Change-Id: Ie3c7a66519b7b8b55e390a512a9ba9e679a8ad0d