Generate new Android.bp file and correct build errors

The following are not yet available in their respective libraries so
attempts to use it in webrtc result in a call to abort():
* libvpx's CONSTRAINED_FROM_ABOVE_DROP constant
* libyuv's I010 buffers

The original webrtc project expects to have third party libraries
checked out in third_party/ and base/third_party/. Added some headers
in those libraries with a single line including the right header from
external/<library>. Updated .gitignore to keep track of said headers.

Bug: 153469641
Test: mm, also built cuttlefish using this library and ran it locally
Change-Id: I2d596942e34093dccc65d4b7b8249b6afc14d31f

modules/video_coding/codecs/vp9/vp9_impl.cc

@@ -768,7 +768,13 @@ int VP9EncoderImpl::InitAndSetControlSettings(const VideoCodec* inst) {
       // Screenshare dropping mode: drop a layer only together with all lower
       // layers. This ensures that drops on lower layers won't reduce frame-rate
       // for higher layers and reference structure is RTP-compatible.
+#if 0
+      // CONSTRAINED_FROM_ABOVE_DROP is not defined in the available version of
+      // libvpx
       svc_drop_frame_.framedrop_mode = CONSTRAINED_FROM_ABOVE_DROP;
+#else
+      abort();
+#endif
       svc_drop_frame_.max_consec_drop = 5;
       for (size_t i = 0; i < num_spatial_layers_; ++i) {
         svc_drop_frame_.framedrop_thresh[i] = config_->rc_dropframe_thresh;