OPS: Update gradle settings and versions.

Related to the issue described here https://stackoverflow.com/questions/60844245/how-solve-could-not-initialize-class-org-codehaus-groovy-reflection-reflectionc.

I encountered this while attempting to build and run the latest master locally on MacOS using the latest version of Andriod Studio (4.0.1).

README.md

@@ -53,7 +53,8 @@ You will now need to either connect an Android device to your computer or run an
 1. Download and run Android Studio
 2. Click on "Open an existing Android Studio Project"
 3. Open `build.gradle` file under `BlueWallet/android/` folder
-4. Android Studio will take some time to set things up. Once everything is set up, go to `Tools` -> `AVD Manager`
+4. Android Studio will take some time to set things up. Once everything is set up, go to `Tools` -> `AVD Manager`.
+    * 📝 This option [may take some time to appear in the menu](https://stackoverflow.com/questions/47173708/why-avd-manager-options-are-not-showing-in-android-studio) if you're opening the project in a freshly-installed version of Android Studio.
 5. Click on "Create Virtual Device..." and go through the steps to create a virtual device
 6. Launch your newly created virtual device by clicking the `Play` button under `Actions` column
 

android/build.gradle

@@ -17,7 +17,7 @@ buildscript {
     }
     ext.kotlinVersion = '1.3.+'
     dependencies {
-        classpath('com.android.tools.build:gradle:3.6.3')
+        classpath('com.android.tools.build:gradle:4.0.1')
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlinVersion"
         classpath 'com.google.gms:google-services:4.3.3'  // Google Services plugin
 
@@ -57,4 +57,4 @@ subprojects {
             }
         }
     }
-}
+}

android/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,6 @@
+#Tue Jul 21 23:04:55 CDT 2020
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-6.0.1-all.zip
 zipStoreBase=GRADLE_USER_HOME
-zipStorePath=wrapper/dists
+zipStorePath=wrapper/dists
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip