Added config system navigation keys.

1. Added config_supportSystemNavigationKeys to config and symbols
2. Added SYSTEM_NAVIGATION_KEYS_ENABLED to Settings.Global
3. Updated PhoneWindowManager to check for the system navigation keys enabled
   flag before expanding or collapsing the notification panel.

Bug 29120082

Change-Id: Ia73078520474ce5a1cbf2256c0d793ed74bbf589