[Android] Enable Java 1.8 compatibility support

It was recently introduced in WebRTC, so we we need to enable it project wide.
As for what features are supportd, see:
https://developer.android.com/studio/write/java8-support.html

android/README.md

 `https://github.com/jitsi/jitsi-maven-repository/raw/master/releases` and the
 dependency `org.jitsi.react:jitsi-meet-sdk:1.9.0` into your `build.gradle`.
 
+Add Java 1.8 compatibility support to your project by adding the following lines
+into your `build.gradle` file:
+
+```
+compileOptions {
+    sourceCompatibility JavaVersion.VERSION_1_8
+    targetCompatibility JavaVersion.VERSION_1_8
+}
+```
+
 ## API
 
 Jitsi Meet SDK is an Android library which embodies the whole Jitsi Meet

android/app/build.gradle

             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
         }
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
 }
 
 dependencies {