Screen Capture: Create ScreenshotInteractor for screenshot requests

Following the SystemUI architecture recommendations an interactor will
be responsible for employing the ScreenshotHelper to handle the business
logic of taking a screenshot. The interactor methods are set up to
support multiple displays, but will default to the default display for
now.

Bug: 424206233
Test: atest ScreenshotInteractorTest
Flag: com.android.systemui.desktop_screen_capture
Change-Id: I0a03dce8c5b94d9d400cd3df99642c12ddabb958