convert UpdateService to JobIntentService

JobIntentService makes it easy to convert an IntentService like UpdateService to work better with Android 8.0's background restrictions. According to @commonsguy:

JobIntentService is there mostly for developers who had been using IntentService (or my WakefulIntentService) in the background and want better results on Android 8.0+ (able to be started from the background, runtime closer to 10 minutes than 1 minute).

This requires bumping min SDK to 14 for android support 26.1.

This builds upon !416 (merged)

Edited Apr 18, 2018 by Hans-Christoph Steiner

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