Remove redundant setContentView to fix screenshot focus

This is essentially the same fix as ag/20620698; it got partially
reverted (unintentionally) as part of a later refactor, probably
due to version shear. The result is that the second call to
setContentView resets the screenshot focus that was just requested,
causing us to no longer receive back key events.

This change removes the redundant call to setContentView (which
should only be called once, in reloadAssets during the constructor)
so that the ScreenshotView retains focus correctly.

Bug: 271497893
Fix: 271497893
Test: manual (by swiping back during a screenshot and verifying that
the screenshot UI is dismissed)

Change-Id: I9ed908e84b8f69e48c900de90d378c4fda0ff7f9