[android_intent] Bump the Flutter SDK min version (flutter#2188)

The previous minimum was too low to support the V2 embedding at compile
time. The latest stable appears to be the oldest version that works with
the plugin.

Author: Michael Klimushyn

packages/android_intent/example/pubspec.yaml

@@ -10,3 +10,6 @@ dependencies:
 # The following section is specific to Flutter.
 flutter:
   uses-material-design: true
+
+environment:
+  flutter: ">=1.9.1+hotfix.2 <2.0.0"

packages/android_intent/pubspec.yaml

@@ -22,4 +22,4 @@ dev_dependencies:
     sdk: flutter
 environment:
   sdk: ">=2.0.0-dev.28.0 <3.0.0"
-  flutter: ">=1.2.0 <2.0.0"
+  flutter: ">=1.6.7 <2.0.0"