Make setRecentsScreenshotEnabled public

Enable setRecentsScreenshotEnabled as a public API to allow apps to
disable the screenshot that is shown in recents.

There's no need for a getter method because it's only set by the app so
it will not be changed by system

Test: SnapshotTaskTests
Fixes: 166729178
Change-Id: I06f1e4fa62ad50c050b6259e04a4af27cd25be67