Fix screenshot back key recognition

We were only requesting focus once, on screenshot view inflation,
and the focus got reset when window.setContentView was called,
causing us to no longer receive key events. This change moves the
setContentView call to only happen once, and requests focus
explicitly for each screenshot.

Test: manual -- took two screenshots in succession and verified that
back gesture worked to dismiss screenshot
Bug: 259490332
Fix: 259490332

Change-Id: Ia0dd38b32f7986088b87f3838bcb05dbbb02238d