UI fixes for tablets

1. Center the clock on Xoom and large tablets. Fix b/5579000

2. Correctly remove the emergency call button if the device (*cough*
   Nakasi *cough*) does not have telephone capability.

Change-Id: Ib7552dc35392a1b9d6c0381c6167949e2b163ddc

res/layout/crypt_keeper_status.xml

@@ -23,6 +23,7 @@
     android:orientation="vertical"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
+    android:layout_gravity="center_horizontal"
 >
 
     <com.android.internal.widget.DigitalClock android:id="@+id/time"
@@ -31,6 +32,7 @@
         android:layout_alignParentTop="true"
         android:layout_alignParentLeft="true"
         android:layout_marginTop="8dip"
+        android:layout_gravity="center"
         android:layout_marginBottom="8dip"
         >