build: switch to using our WebRTC build from Maven Central

This change is mostly for F-Droid (see
https://snikket.org/blog/fdroid-security-warning/ for some context).

F-Droid "trusts" dependencies published in Maven Central, so that's what
I've done (see https://github.com/snikket-im/webrtc-android ). From this
version, F-Droid will be able to stop using the outdated WebRTC last published
by Google.

In the future F-Droid may build and provide their own WebRTC:
https://gitlab.com/fdroid/admin/-/issues/364

build.gradle

@@ -77,7 +77,7 @@ dependencies {
 
     implementation 'com.google.guava:guava:30.1.1-android'
     quicksyImplementation 'io.michaelrocks:libphonenumber-android:8.12.18'
-    implementation fileTree(include: ['libwebrtc-m92.aar'], dir: 'libs')
+    implementation 'org.snikket:webrtc-android:107.0.0'
 }
 
 ext {