title: error: package androidx.core.app does not exist when running flutter blue plugin
body: <p>I'm working on Flutter and use Flutter blue plugin. 
  Everything ran well at the beginning but suddenly the program can't perform build because of flutter blue plugin version.</p>
  
  <pre><code>    dependencies:
        vector_math: any
        path_provider: ^1.3.0
        flutter:
          sdk: flutter
        flutter_map:
        flutter_blue: ^0.6.3+1
        cupertino_icons: ^0.1.2
  </code></pre>
  
  <p>this is the error message</p>
  
  <pre><code>    C:\flutter\.pub-cache\hosted\pub.dartlang.org\flutter_blue-0.6.3+1\android\
      src\main\java\com\pauldemarco\flutter_blue\FlutterBluePlugin.java:43:
      error: package androidx.core.app does not exist import androidx.core.app.ActivityCompat;
      ^
      error: package androidx.core.content does not exist
      import androidx.core.content.ContextCompat;
  </code></pre>