Use default rtp parameters to init wrappers in iOS

Before these changes default initialized iOS wrappers
around various RTP*Parameters types had their own
default values of nonnull values, which did not always
matched default values from native code, which then causes
override of default native values, if library user didn't
specified it's own initialization.
After these changes default initialization of iOS wrappers
uses default property values from default initialized
native types.

Bug: None
Change-Id: Ie21a7dc38ddc3862aca8ec424859c776c67b1388
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215220
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33731}

sdk/objc/api/peerconnection/RTCRtpEncodingParameters.mm

@@ -26,12 +26,13 @@
 @synthesize networkPriority = _networkPriority;
 
 - (instancetype)init {
-  return [super init];
+  webrtc::RtpEncodingParameters nativeParameters;
+  return [self initWithNativeParameters:nativeParameters];
 }
 
 - (instancetype)initWithNativeParameters:
     (const webrtc::RtpEncodingParameters &)nativeParameters {
-  if (self = [self init]) {
+  if (self = [super init]) {
     if (!nativeParameters.rid.empty()) {
       _rid = [NSString stringForStdString:nativeParameters.rid];
     }