Minor Alt-TAB / Recent Apps Dialog improvements.

Alt-TAB should have different semantics from the APP_SWITCH key
or long-press on HOME.  Accordingly, remove the fallback action
for Alt-TAB and initiate the task switching behavior directly
in the policy.

Modified RecentApplicationsDialog to be more precise about the
initial modifiers that it considers to be holding the dialog.

The dialog is now dismissed by a second press on the APP_SWITCH
key or by a second long press on HOME.

Change-Id: Idf4d803f51103819057cb655ff3b770b7729e4be