android: re-installation of native backend

[bruno-garcia]

Android reinitialization of the sentry-native backend after C# init to compensate Unity taking the signal handler and not calling back into Sentry's one.

Additionally, other changes happened in the process that were hard to split into different PRs:

• Adding breadcrumb level for Android (replacing fix: android crumb level #338)
• Aligned behavior (mainly logging) between iOS and Android C# bridge
• Moved any logic/complex code away from the SentryInitialization class since we don't get that to compile with the solution. There's now a single method to configure the native layer through C# and less code to document on the 'manual initialization'
• Enabled NDK scope propagation by default (we can possibly add an opt-out in the future if this becomes an issue in terms of overhead)
• Unit tests where possible
• SendMessage button changed to a SentrySdk.CaptureMessage since Debug.Log only tracks breadcrumbs
• Changed the CaptureExceptionToString which was used for testing the parsing code originally to a SetUser so we can have a sample of scope propagation: User set in C# should end up in errors in Cocoa, Java, C:

Example event in Sentry:

[bitsandfoxes]

Thanks for the additional cleanup!