Clean up camera flipping, handle having missing cameras.

Did a refactor to better organize the camera flipping code. Also, I
wanted to make sure we handle the cases where the user doesn't have two
cameras (or no cameras, for that matter). In these cases, we just don't
show the appropriate buttons.

res/drawable/webrtc_camera_front_button.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
+    <item android:top="5dp"
+          android:left="5dp"
+          android:right="5dp"
+          android:bottom="5dp"
+          android:drawable="@drawable/quick_camera_front"/>
+</layer-list>

res/drawable/webrtc_camera_rear_button.xml

@@ -1,6 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <layer-list xmlns:android="http://schemas.android.com/apk/res/android">
-    <item android:drawable="@drawable/webrtc_control_background"/>
     <item android:top="5dp"
           android:left="5dp"
           android:right="5dp"

res/layout/webrtc_call_controls.xml

@@ -41,6 +41,7 @@
                 android:id="@+id/camera_flip_button"
                 style="@style/WebRtcCallCompoundButton"
                 android:contentDescription="@string/redphone_call_controls__flip_camera_rear"
-                android:background="@drawable/webrtc_camera_flip_button"/>
+                android:background="@drawable/webrtc_camera_rear_button"
+                android:visibility="gone"/>
 
 </merge>

res/values/strings.xml

@@ -995,7 +995,7 @@
     <!--- redphone_call_controls -->
     <string name="redphone_call_card__signal_call">Signal Call</string>
     <string name="redphone_call_controls__mute">Mute</string>
-    <string name="redphone_call_controls__flip_camera_rear">Use rear camera</string>
+    <string name="redphone_call_controls__flip_camera_rear">Switch Cameras</string>
     <string name="redphone_call_controls__signal_call">Signal Call</string>
 
     <!-- registration_activity -->