Optimize android's Gradle settings and clean up mobile source code (#240)

* optimize android side gradle settings

* android minsdk back to 21

* remove unused package, update linter and fix lint error

mobile/android/app/build.gradle

@@ -81,5 +81,4 @@ flutter {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
-    implementation 'com.android.support:multidex:1.0.3'
 }