Dont acquire prox sensor if there are no calls.

Bug:17323179
Change-Id: Ibc022b1ff9c4a80a2e53a3af74a1659b8047386c
diff --git a/src/com/android/telecomm/ProximitySensorManager.java b/src/com/android/telecomm/ProximitySensorManager.java
index 287c6d4..7b8773d 100644
--- a/src/com/android/telecomm/ProximitySensorManager.java
+++ b/src/com/android/telecomm/ProximitySensorManager.java
@@ -52,6 +52,11 @@
      * Turn the proximity sensor on.
      */
     void turnOn() {
+        if (CallsManager.getInstance().getCalls().isEmpty()) {
+            Log.w(this, "Asking to turn on prox sensor without a call? I don't think so.");
+            return;
+        }
+
         if (mProximityWakeLock == null) {
             return;
         }