Correct the string resource for tooltip_popup_title

Since "tooltip" is defined as "small popup window", "Tooltip Popup"
is redundant. Replacing it with just "Tooltip".

Bug: 33257550
Test: "adb shell dumpsys window windows | grep Tooltip"
with a tooltip open, check the window title.

Change-Id: Id06e26051e4b1afcf003723d85751b51bcafa93b