More minor cleanups to ScreenshotRequest

* Adds screenshotType into the request instead of relying on the
Message it is sent in (msg.what).

* Adds a constructor which accepts type, source and topComponent

* Adds more @IntDef annotations to `int` constructor args

Bug: 231957192
Test: atest ScreenshotHelperTest
Change-Id: If4bf8ef1b889e4e7b2fabf195a538a13664cce12