Feature/foreground service

[MGasztold]

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This pull requests solves issues with Android versions >=8 in which service is killed by OS when app is in background or after screen is turned off. The price to pay for this is the notification that has to be permanently visible in the app's status bar.

The API I proposed let's developer start MQTT service as foreground service and provide its own Notification object. In the same time it is totally optional.

I also got rid of legacy support library dependency in favor of latest androidx dependency.

This pull requests tries to solve the following issues:
#416 #332 #281 #274 #226

[alexwijoyo]

Hi, what is the status of this pull request? I need the foreground service capabilities. Thanks.

[MGasztold]

@alexwijoyo Hello, I am successfully using it in my project. I do not know what is left
for it to be merged to develop. Probably it awaits a review from somebody
with the rights to eventually apply it.

[edu1525]

Hello, I really need this pull request in my project, I'm having some problems with foreground services.

[MGasztold]

Anybody knows whose attention should I get in order to make this pull request be merged?

[alexwijoyo]

@MGasztold I will try to incorporate your fork library source to my project. Thanks.

[thanksmister]

Is this going to get merged?