Add public API for crash-on-launch detection

[fractalwrench]

Goal

Adds the public API for detecting whether an app crashed on launch.

Changeset

Made the following interfaces are now available as part of the public API, adding JavaDoc as appropriate. Note that this simply adds the API - this will be hooked up to the actual functionality in separate changesets.

class Bugsnag {
    static LastRunInfo getLastRunInfo();
    static void markLaunchCompleted();
}

class Client {
    LastRunInfo getLastRunInfo();
    void markLaunchCompleted();
}

class Configuration {
    Long launchDurationMillis;
    Boolean sendLaunchCrashesSynchronously;
}

class AppWithState {
    Boolean isLaunching = null;
}

final class LastRunInfo {
    final int consecutiveLaunchCrashes;
    final boolean crashed;
    final boolean crashedDuringLaunch;
}

Additionally loaded sendLaunchCrashesSynchronously from the manifest, and verified that app.isLaunching is serialized to JSON.

Testing

Added unit test coverage

[tomlongridge]

LGTM - one small wording suggestion