Fixed a bug with the animation of the notification background

The position of the background could be 0 and therefore the
background could fly over the whole screen.
We're now positioning the top section at the start
of the first visible section.

Fixes: 137885070
Test: atest SystemUiTests
Change-Id: I6720b570f5cd209249b90681cdf6c6a56cf0194e
1 file changed