Multi-OS Engine support + moving of generic Java code to commons

[augustweinbren]

Description

This PR does not modify any existing behavior. It consists of the following changes:

• Support for Multi-OS Engine added to build.gradle
• commons(to consist of shared iOS/android code) and ios modules created.
• Boiler plate code for ios module automatically generated.
• Models package + parts of API package which do not use Android libraries moved to commons module.

Related Issues

[#90] (note: this PR should not close the issue.)

[augustweinbren]

Although the Android app builds successfully, I am currently seeing the following error message when running the app.

E/FirebaseCrashlytics: Settings request failed.
java.io.FileNotFoundException: https://firebase-settings.crashlytics.com/spi/v2/platforms/android/gmp/1:460393230596:android:a4512316c4b07001132051/settings?instance=d2ce15d64f057f5a768d8a0187b20776f9e4e11b&build_version=581&display_version=0.7.0&source=1
at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:255)
at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:211)
at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:30)
at com.google.firebase.crashlytics.internal.network.HttpGetRequest.execute(HttpGetRequest.java:80)
at com.google.firebase.crashlytics.internal.settings.network.DefaultSettingsSpiCall.invoke(DefaultSettingsSpiCall.java:113)
at com.google.firebase.crashlytics.internal.settings.SettingsController$1.then(SettingsController.java:199)
at com.google.firebase.crashlytics.internal.settings.SettingsController$1.then(SettingsController.java:192)
at com.google.android.gms.tasks.zzo.run(com.google.android.gms:play-services-tasks@@17.2.0:2)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
at com.google.firebase.crashlytics.internal.common.ExecutorUtils$1$1.onRun(ExecutorUtils.java:64)
at com.google.firebase.crashlytics.internal.common.BackgroundPriorityRunnable.run(BackgroundPriorityRunnable.java:27)
at java.lang.Thread.run(Thread.java:923)

[hpsaturn]

Thanks @augustweinbren I'm going to test and review it..

[hpsaturn]

Thanks for your PR, for now it build ok and works ok in a first review, I have only one request. For other side I changed the pull request target branch to devel.