Enabling multidex support

When writing large apps or making use of large plugins, you may encounter
Android’s dex limit of 64k methods when targeting a minimum API of 20 or
below. This may also be encountered when running debug versions of your app
via flutter run that does not have shrinking enabled.

Flutter tool supports easily enabling multidex. The simplest way is to
opt into multidex support when prompted. The tool detects multidex build errors
and will ask before making changes to your Android project. Opting in allows
Flutter to automatically depend on androidx.multidex:multidex and use a
generated FlutterMultiDexApplication as the project’s application.

Note:
  Multidex support is natively included when targeting Android SDK 21 or later.
  However, it isn’t recommended to target API 21+ purely to resolve the multidex issue
  as this might inadvertently exclude users running older devices.