fix(android): remove dead buildscript block and kotlin_version reference (#1567)

* fix(android): remove dead buildscript block and kotlin_version reference

* fix(android): use fully-qualified kotlin plugin id to match settings.gradle

mobile/android/app/build.gradle

@@ -1,6 +1,6 @@
 plugins {
     id "com.android.application"
-    id "kotlin-android"
+    id "org.jetbrains.kotlin.android"
     id "dev.flutter.flutter-gradle-plugin"
 }
 
@@ -99,6 +99,3 @@ flutter {
     source '../..'
 }
 
-dependencies {
-    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
-}