Make the libaom AV1 encoder optional in RTCDefaultVideoEncoderFactory.

This is a partial revert of: https://webrtc-review.googlesource.com/c/src/+/262814

skip_shebang_check=True

bug: webrtc:13573
Change-Id: I08282df91446efa79ca86cc8dfe3b6daeb4eec9a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/264543
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37074}

sdk/objc/api/video_codec/RTCVideoEncoderAV1.mm

@@ -15,15 +15,28 @@
 #import "RTCVideoEncoderAV1.h"
 #import "RTCWrappedNativeVideoEncoder.h"
 
-#include "modules/video_coding/codecs/av1/libaom_av1_encoder.h"
+#if defined(RTC_USE_LIBAOM_AV1_ENCODER)
+#include "modules/video_coding/codecs/av1/libaom_av1_encoder.h"  // nogncheck
+#endif
 
 @implementation RTC_OBJC_TYPE (RTCVideoEncoderAV1)
 
 + (id<RTC_OBJC_TYPE(RTCVideoEncoder)>)av1Encoder {
+#if defined(RTC_USE_LIBAOM_AV1_ENCODER)
   std::unique_ptr<webrtc::VideoEncoder> nativeEncoder(webrtc::CreateLibaomAv1Encoder());
-
   return [[RTC_OBJC_TYPE(RTCWrappedNativeVideoEncoder) alloc]
       initWithNativeEncoder:std::move(nativeEncoder)];
+#else
+  return nil;
+#endif
+}
+
++ (bool)isSupported {
+#if defined(RTC_USE_LIBAOM_AV1_ENCODER)
+  return true;
+#else
+  return false;
+#endif
 }
 
 @end