Improvements to multi select

* selection bar now uses an <- instead of X icon
* FAB animates away when entering either selection or search mode
* allow selection mode to be entered once already in search mode

Demo: https://drive.google.com/a/google.com/file/d/0B-vjphxTdjuANERMUDF6Y2NyQXc/view?usp=sharing

Bug: 19549465
Change-Id: I657bf564c8e6cdfca6f7779918c4807e09e78d9b