* Fix Android builds so not `assert(false)` immediately on startup
due to different JNI and Android-JNI `FindClass` requirements.
* Fix Java's `ChannelManagerConstructor` to properly pass the
`NetworkGraph` to the `BackgroundProcessor` to persist it when P2P
gossip sync is enabled.
* Change Java's `ChannelManagerConstructor` to take lower-level
signing interfaces rather than a `KeysManager`, allowing custom
types or a `PhantomKeysManager` to be used.
* Fix trait passing to hit an immediate `NullPointerException` rather
than SEGFAULTing in native code.
* Switch to building macOS binaries on Linux, which makes them a few
bytes short of deterministic.
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