Add option to return a null Dagger graph

This allows processes that do not utilize the SysUI Dagger graph to
avoid having to subclass and instantiate it.

This change also makes the Screenshot cross-profile service take
advantage of this change.

Test: Turn on "Enable RequestProcessor" and "Enable Work Profile
Screenshots Policy" SysUI flags
Test: Take a screenshot of a work profile app
Test: Observe successful work profile screenshot
BUG: 259469497

Change-Id: I1a8c62ba079d90e575cab2a0e14b3f1466e1952c