Give up “startAssist()”if the object “mView” of class “AssistOrbContainer” is null.

Give up “startAssist()”if the object “mView” of class “AssistOrbContainer” is null.

If device has physical NavigationBar,will not call method
"com.android.systemui.assist.AssistManager.onConfigurationChanged()",
cause the object "mView" is null. When input command "input keyevent 219",
will call methed "com.android.systemui.assist.AssistManager.startAssist()",
but object "mView" is null , result in JAVACHRASH.
So we need give up "com.android.systemui.assist.AssistManager.startAssist()" for physical NavigationBar.

Change-Id: I5c74643c9d99dc56d80a54eea6e798365b044c42
Tracked-On: https://jira01.devtools.intel.com/browse/OAM-13670
Signed-off-by: xiaowa1x <xiaox.wang@intel.com>
Reviewed-on: https://android.intel.com:443/454888