Move SaveImage-, DeleteImage- to separate files

SaveImageInBackgroundTask and DeleteImageInBackgroundTask were
recently moved to inner classes of GlobalScreenshot to make the
latter injectable. This change moves them instead to separate
files, to make GlobalScreenshot shorter and more easily
comprehensible.

Test: tested old and new flows; no changes other than moving the
classes and formatting
Bug: 137153302

Change-Id: Iec27fdef541809c24e194fe377e3fad3dc19e213