Fix 3386135: Make recent apps scrollable with more entries

Summary of changes:

Switched to using a ListView to contain recent apps.

Increased DISPLAY_TASKS to show up to 20 recent applications.

Moved some parameters to dimens to make them more customizeable
by partners and by device and configuration.

Adjusted size of composited thumbnail to accomodate new thumbnail
background asset.

Hide IME buttons in TabletStatusBar while RecentApps panel is showing.

Use new drawable for selection.

Restore list position on orientation change.

Change-Id: If8c26869b7e70f82c097870c6c1b928b959a4685