Fix calculating Notification amountInShelf

ExpandableView#isInShelf() returns true if a Notification is fully
scrolled into the shelf.
After the change I2735055775632649c418880322e7e197acfd84e7 we have left a check for isInShelf, for calculating the amountInShelf fraction for a notification that is partially scrolled into the shelf. This prevented us from doing an alpha and a translateY animation on the shelf icons.

Fixes: 319642729
Test: atest NotificationShelfTest
Test: post some notifications, open the shade, and scoll them into the
shade
Flag: NONE

Change-Id: Id8d6ff7e645e6a83a650ff00a4b026aec87ae6ec