Fix CMHome related bugs.

1. If "Search Panel" is enabled, when dragging an app icon to a new
   screen, the 'glow' effect for the next page will highlight the entire
   height of the screen instead of just the workspace view height as
   expected. This bug exists in launcher3 and GEL. Fixed here.
2. Older versions of the Google Search app do not respond to
   Intent.ACTION_ASSIST and can crash Trebuchet when it tries to launch
   Google Now that way. Instead, use searchManager.globalSearchActivity(),
   which matches the method we use to detect Google Now's presence anyway
   (+ being "search app" agnostic!)

Change-Id: I5e5b3504272e2a2ca656accc32164d1ae9e52077
(cherry picked from commit 9381658f0b23d2e91762527f31a24dec4cd92883)