Fix double call to ScreenshotController

When SCREENSHOT_REQUEST_PROCESSOR is enabled, the request is
dispatched via RequestProcessor callback. This should return
without following the original code path without the flag
enabled.

Added a test case to ensure the request is only forwarded once.

Bug: 159422805
Test: atest TakeScreenshotServiceTest
Change-Id: I8df2dd8e97e1aadde9bbeb141fe29940f9b6a5ba