Make mScreenshotController final and @NonNull

Simplifies the code by removing the need to null check altogether.
Until the service is onDestroy()'d, it should be capable of handling
requests, since it may be rebound before this. Nulling the field
in onDestroy is also unneccesary. Making the field final removes
the risk of NPE entirely.

Note: this also allows mScreenshot.onDestroy() to actually be called,
as is this is never executed.

(This was part of my change introduced in commit b283231)

Test: manual; Take screenshots, dismiss and immediately take another
Test: monkey test, see bug
Fix: 242881882
Change-Id: I2a25fd4311f2fc7d5802a5d2a9bd8af64f845868