SearchView improvements per design.

- X is visible only if there is text, or we need a way to close a
search field that is iconified by default.
- Search dialog (legacy) has a back button to the left.
- Hitting X on a non-focused search view will bring it into focus
and show the keyboard if necessary.

Change-Id: I5a30bb08adcf84639a922a9e13be1d1562f714e6