Add FLAG_ALT_FOCUSABLE_IM to screenshot window.

Avoid interacting with the IME when we don't need it in the screenshot
window.

Bug: 158664238
Test: Verify that IME is not dismissed when triggering screenshot.
      Verify that UI exploration with talkback still works.
Change-Id: Ie13b1d01ce54862b39df646d57fcfcee1e74f8d6