Handle iOS devices with no rear-facing camera

Devices exist (specifically the 16GB 5th generation iPod Touch) that do not have
a rear-facing camera.

This CL:
- Adjusts RTCAVFoundationVideoCapturerInternal so initialization doesn't fail
because a rear-facing camera doesn't exist, but still logs a warning
- Provides a check for whether or not a rear-facing camera can be used (useful
for toggling UI elements)
- Turns an attempt to switch to the rear-facing camera into a no-op with a
warning.

BUG=

Review URL: https://codereview.webrtc.org/1416303003

Cr-Commit-Position: refs/heads/master@{#11992}

webrtc/api/objc/RTCAVFoundationVideoSource.h

@@ -28,6 +28,9 @@ NS_ASSUME_NONNULL_BEGIN
 - (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
                     constraints:(nullable RTCMediaConstraints *)constraints;
 
+/** Returns whether rear-facing camera is available for use. */
+@property(nonatomic, readonly) BOOL canUseBackCamera;
+
 /** Switches the camera being used (either front or back). */
 @property(nonatomic, assign) BOOL useBackCamera;