transport: Don't put a capsfilter after the audio decoder and link the depayloader

We can't just force the audio decoder to output something specific, this will
fail with most audio decoders out there... and if conversion is required we
will do that later already.

Also make sure the pad that is ghosted is the decoded_valve and not the
capsfilter, which is incorrect.
OwrMediaRenderer: fix refcount bug in owr_media_renderer_set_source
sdp: Allow a=rtcp address to be optional

In accordance with RFC 3605
bridge: Don't assume mdesc.rtcp exists when checking mdesc.rtcp.mux
domutils: Use the expected variable name
webrtc_js: Avoid assigning the same property twice
peerhandler: Set the TURN server transport type from the iceServer URL

In accordance with RFC 7065
Add test utilities

Run ./out/<platform>/bin/test-self-view after building to run a simple
audio/video capture and output test.

Run ./out/<platform>/bin/test-send-receive after building to run a test
with one sender and one receiver in the same application.
Update scripts

Various build fixes.
added the possibility to export symbols for specific platforms from libopenwebrtc.exp

Syntax: <symbol> [if <host> [or <other host>]...]
added OwrWindowHandle gir type + annotations to owr_window_registry
added gen_jni.py, which generates Java bindings for Android using GIR and JNI
added OwrBridge JNI implementation
modified build to generate, compile, and link JNI files when building for Android
compile generated Java classes and create openwebrtc.jar and openwebrtc_bridge.jar when building for Android