Implemented version checking

[ali-thowfeek]

Purpose

The purpose of this PR is to fix #179

Known issue

The code runs twice, as the widget gets initialized twice (even before the implementation of version checking). Thus two dialogs appear on top of the other.

It is another issue mentioned here #192

Work around

The issue is likely to be because of the the implementation of the DropdownBanner package. Because the following seems to fix it.

Change this:

MaterialApp(
//....
//....
home: DropdownBanner(
        child: HomeScreen(),
        navigatorKey: navigatorKey,
      ),
)

To:

MaterialApp(
//....
//....
home: HomeScreen(),
)

Notes

@dewmal Before merging, Please create and assign respective values for the following in the firebase console remote config:
android_app_builder_number_Key
ios_app_builder_number_key