Improve IAP handling

[khaykov]

This PR fixes some ANR's that result from billing service. Despite following Google's guidelines, we are getting ANR's when initializing billing service, so I moved some calls to background thread, and added a backoff logic.

Backoff logic:

• Comment out this line.
• Add debuggable true to the release config. Should look like this:

release {
            debuggable true
            minifyEnabled true
            shrinkResources false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }

• Launch app in release configuration.
• In the logcat, confirm that you see IapViewModel I Billing response OK
• Open device settings, navigate to Apps, and find Play Store.
• Clear Play Store storage.
• Check logcat, and confirm that you see IapViewModel I Retrying billing service connection after 1000 MS delay
• Do this again, and confirm that same message with 2000 MS delay.
• etc.

Afte this:

• Confirm that billing flow works by purchasing sustainer plan.

[wpmobilebot]

You can test the changes on this Pull Request by downloading an installable build (simplenote-android-installable-build-pr1603-eaed77e-0189db1f-571c-42ff-abf5-c589fee75e82.apk), or scanning this QR code:

[danilo04]

Hi @khaykov, this is working as expected. Great job here!!