Updated android install instructions (#6485)

Copied over from playground app.
Sets the version only once and then reuses the variable

website/docs/docs-Installing.mdx

@@ -201,8 +201,8 @@ buildscript {
     compileSdkVersion = 26
     targetSdkVersion = 26
     supportLibVersion = "26.1.0"
-+   RNNKotlinVersion = "1.3.61" // Or any version above 1.3.x
-+   RNNKotlinStdlib = "kotlin-stdlib-jdk8"
++   kotlinVersion = "1.3.72" // Or any version above 1.3.x
++   RNNKotlinVersion = kotlinVersion
   }
   repositories {
         google()
@@ -211,7 +211,7 @@ buildscript {
 +        mavenCentral()
   }
   dependencies {
-+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.61" // Or whatever Kotlin version you've specified above
++        classpath("org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion")
 +        classpath 'com.android.tools.build:gradle:3.5.3' // Or higher
 -        classpath 'com.android.tools.build:gradle:2.2.3'
   }
@@ -362,4 +362,4 @@ dependencies {
     implementation "com.android.support:appcompat-v7:${rootProject.ext.supportLibVersion}"
 }
 
-```
+```