Optimize implementation of internal state change observers

[fractalwrench]

Goal

Optimizes the implementation of the internal system used for notifying observers of state changes.

Changeset

This new solution avoids the performance disadvantages of the previous solution:

• The StateEvent message is lazily created which avoids object instantiation if no observers are registered
• BaseObservable avoids creating a Vector by replacing update() with onStateChange() (this also grants additional type safety)
• BaseObservable uses CopyOnWriteArrayList which avoids the need for synchronization when adding/removing observers. This was seen as acceptable as observers will be added/removed very rarely, whereas the collection will be iterated over many times

The remaining changes entail updating the rest of the code and the tests to use the new method signatures.

Testing

Updated existing unit test coverage and relied on existing tests.

Performance

Profiled app to confirm there was no performance regression, and also checked CI stats.

[bugsnagbot]

Android notifier sizes

Format	Size impact of Bugsnag (kB)	Size impact of Bugsnag when Minified (kB)
APK	1513.92	1409.88
arm64_v8a	406.2	299.71
armeabi	385.72	279.22
armeabi_v7a	369.34	262.85
x86	443.05	336.55
x86_64	426.67	320.18

Generated by 🚫 Danger

[lemnik]

LGTM