Allow sticky immersive mode by mouse hover.

When the app enters into the immersive mode, swipe down from the
top edge makes immersive_sticky (showing transient status bar),
but this is hardly done by mouse. As is discussed in the bug,
we want to make the same effect for mouse hover at the very top
line of the screen for 500msec.

Same applied to navigation bar with the bottom of the screen.

Bug: 20093789
Change-Id: If4484b5bcee567ea89c28ed06807564cb8f9ccef