Java client mode sessions

Android has client mode but not the Java core package.
We can refactor this out to be done in the core Java as it was done in .NET. Exposing a 'pause and resume' method that tracks how long the sessions was in the background and closes/reopens automatically.

Client-mode session support: App Lifecycle. Starts a session when the process starts, ends when it exits. (similar to sentry-native and .NET). Potential JavaFX integration could rely on this. Plus if supported foreground/background events would close/restart session