Add Snowflake pluggable transport.

bramble-android/build.gradle

...org/briarproject/bramble/plugin/tor/AndroidTorPlugin.java

bramble-android/witness.gradle

...riarproject/bramble/plugin/tor/CircumventionProvider.java

...project/bramble/plugin/tor/CircumventionProviderImpl.java

...n/java/org/briarproject/bramble/plugin/tor/TorPlugin.java

bramble-core/src/main/resources/bridges

bramble-core/src/main/resources/snowflake-params

...project/bramble/plugin/tor/CircumventionProviderTest.java

bramble-java/build.gradle

.../java/org/briarproject/bramble/plugin/tor/BridgeTest.java

bramble-java/witness.gradle

briar-android/witness.gradle

build.gradle