Minor fix for iOS 12.2

[yurymkomarov]

• fix iOS 12.2 crash ("_alwaysRunsAtForegroundPriority" -> "alwaysRunsAtForegroundPriority")
• update UIBackgroundModes default value (to prevent rejects from AppStore)

[yurymkomarov]

@katzer , please review, merge and update package for comfortable use for another users

[ellecito]

It works. Thanks!

[ghenry22]

Note that this will break on versions of iOS before 12.2 due to the change in config key. You really need to include a check for version 12.2 or greater and use the new setting, else use the old setting so that backward compatibility is maintained.

I submitted a similar fix to the ionic wkwebview plugin so you could just cut and paste off that if you want a quick fix. It's just the addition of an if/else.

[qkdreyer]

See #447