Add option to disable screen security

Allow WindowManager.LayoutParams to be toggled between being FLAG_SECURE and default to allow for user screenshots.

See WhisperSystems/TextSecure#788

Also might have `git strip-space`-ed each file... sorry about the extra lines in the diff. Tested on an HTC One. Might want to force it to upate the display when the option is toggled as well, but I wasn't sure (as it could be accidentally toggled and you might not want it to update right away).