Fix crash in -[BugsnagSystemState sync]

[nickdowell]

Goal

The crash was reported against -[BugsnagSystemState sync] + 203 (BugsnagSystemState.m:203) which is retaining the currentLaunchState property.

- (void)sync {
    NSDictionary *state = self.currentLaunchState;
    NSError *error = nil;
    ...

This crashed because currentLaunchState was declared nonatomic but is being set and read from different threads.

Changeset

• Made the currentLaunchState property atomic - this ensures the returned object will not be deallocated by a setting thread before the getter completes.
• Marked other nonatomic properties as readonly where appropriate to avoid similar issues being introduced in the future.

Testing

N/A