Quality improvements needed in screenshot-view nav (#4)

"1) Right and left arrows do not always work"
----i enlarge the arrow's size, now it should works
"2) Sometimes after tapping a right or left arrow, the next/previous screenshot loads twice."
----fixed
3) now the android back button to go back to screenshot view works. a bar was added to tray screen, now we can swipe up from off screen, the bar or above the bar (if there is empty space) to close the tray screen.