Prevent ConcurrentModificationException when setting redactedKeys

[fractalwrench]

Goal

Prevents a ConcurrentModificationException from being thrown when redactedKeys is set on the Metadata of an Event. This can manifest as a crash when notify() is called concurrently.

Changeset

• Converted ObjectJsonStreamer to use var Set<String> rather than val MutableSet<String>, as this avoids the problem of mutating a collection concurrently
• Converted redactedKeys to a property on Metadata rather than a property and function, as this is more consistent
• Removed ObjectJsonStreamer from the constructor as there is no need for it to be injectable

Testing

Added a unit test to verify that when redactedKeys is set on Metadata a crash does not occur. Additionally added an integration test which calls notify() concurrently to confirm that no further crashes occur with the default configuration.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1437.59	1358.81
arm64_v8a	369.25	287.33
armeabi	348.77	266.85
armeabi_v7a	328.29	250.47
x86	406.1	324.19
x86_64	389.72	307.81

Generated by 🚫 Danger

[tomlongridge]

LGTM - one request for an extra assertion.