Screen capture: Esc keypress closes the window

Implements the key event handling on the top-level Composable by making
the component focusable. The window and its view were not chosen to
handle the key events as making them focusable led to undesired focus
shadows and behavior.

Bug: 438561227
Test: Manual
Flag: com.android.systemui.large_screen_screencapture
Change-Id: I97f3f7bbc91db15cb390b35c350bb6a1888d80ab