update gradle and androidx

example/android/app/build.gradle

 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
-    compileSdkVersion 27
+    compileSdkVersion 28
 
     lintOptions {
         disable 'InvalidPackage'
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.example.example"
         minSdkVersion 16
-        targetSdkVersion 27
+        targetSdkVersion 28
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
 
 dependencies {
     testImplementation 'junit:junit:4.12'
-    androidTestImplementation 'com.android.support.test:runner:1.0.2'
-    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
+//    androidTestImplementation 'com.android.support.test:runner:1.0.2'
+//    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
 }

example/android/build.gradle

     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:3.1.2'
+        classpath 'com.android.tools.build:gradle:3.3.1'
     }
 }
 

example/android/gradle.properties

 org.gradle.jvmargs=-Xmx1536M
+android.enableJetifier=true
+android.useAndroidX=true

example/android/gradle/wrapper/gradle-wrapper.properties

 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.2.1-all.zip

pubspec.lock

   photo_manager:
     dependency: "direct main"
     description:
-      name: photo_manager
-      url: "https://pub.flutter-io.cn"
-    source: hosted
-    version: "0.2.1"
+      path: "."
+      ref: "8ef0bf9047cdd86e2331f0db8d15c4c50f35610d"
+      resolved-ref: "8ef0bf9047cdd86e2331f0db8d15c4c50f35610d"
+      url: "https://github.com/CaiJingLong/flutter_photo_manager.git"
+    source: git
+    version: "0.3.0"
   quiver:
     dependency: transitive
     description:

pubspec.yaml

   flutter:
     sdk: flutter
 
-  photo_manager: ^0.2.1
-  # photo_manager:
-  #   git:
-  #     url: https://github.com/CaiJingLong/flutter_photo_manager.git
-  #     ref: 29d844b2c4594d7e524d0169658bc4f3b9ef2ddc
+  # photo_manager: ^0.2.1
+  photo_manager:
+    git:
+      url: https://github.com/CaiJingLong/flutter_photo_manager.git
+      ref: 8ef0bf9047cdd86e2331f0db8d15c4c50f35610d
   # photo_manager:
   #   path: ../flutter_photo_manager