[android_alarm_manager] Update minimum Flutter version to 1.12.0 (#2327)

packages/android_alarm_manager/pubspec.yaml

diff --git a/packages/android_alarm_manager/pubspec.yaml b/packages/android_alarm_manager/pubspec.yaml
index 9239fa4..55e1605 100644
--- a/packages/android_alarm_manager/pubspec.yaml
+++ b/packages/android_alarm_manager/pubspec.yaml
@@ -22,5 +22,4 @@
 
 environment:
   sdk: ">=2.0.0-dev.28.0 <3.0.0"
-  # TODO(bkonyi): set minimum Flutter version to next stable release
-  flutter: ">=1.2.0 <2.0.0"
+  flutter: ">=1.12.0 <2.0.0"