Split SCREENSHOT_METADATA into two different flags.

The screenshot metadata work consisted of two things:
1. Flag-guarded refactors to support metadata handling.
2. The actual fetching and sending of the metadata URL itself.

The second part we've chosen to leave in teamfood for an extended time,
but some other features were expecting to launch along with the
flag-guarded refactor, so we need to flag and launch that separately.

No behavior change in this CL, just splitting one teamfood flag into two
teamfood flags.

Bug: 268484562
Bug: 268484915
Test: atest TakeScreenshotServiceTest
Change-Id: I4654b86206f77a589c1f95565ddb0622c0ce5849