Code cleanup of ScreenshotRequest and usages

Make constructor visible for tests
Make all fields final (no setters, so already effectively final)
Remove mHasStatusBar and mHasNavBar (unused)
Remove overload 'takeScreenshot' method without a source param
Added missing @IntDef annotations to parameters

Bug: 231957192
Test: atest ScreenshotHelperTest SystemActionPerformerTest
Change-Id: Ief43bd0ba50dafac720ae18d00c3c99b8d31e2e1