Updates gradle to 6.2.1 and adds NDK image.

Dockerfile

@@ -1,4 +1,4 @@
-FROM circleci/android:api-29
+FROM circleci/android:api-29-ndk
 
 USER root
 
@@ -15,12 +15,12 @@ RUN apt-get install -y curl \
 # Download and install Gradle
 RUN \
   cd /usr/local && \
-  curl -L https://services.gradle.org/distributions/gradle-5.6.3-bin.zip -o gradle-5.6.3-bin.zip && \
-  unzip gradle-5.6.3-bin.zip && \
-  rm gradle-5.6.3-bin.zip
+  curl -L https://services.gradle.org/distributions/gradle-6.2.1-bin.zip -o gradle-6.2.1-bin.zip && \
+  unzip gradle-6.2.1-bin.zip && \
+  rm gradle-6.2.1-bin.zip
 
 # Export some environment variables
-ENV GRADLE_HOME=/usr/local/gradle-5.6.3
+ENV GRADLE_HOME=/usr/local/gradle-6.2.1
 ENV PATH=$PATH:$GRADLE_HOME/bin
 
 # install Firebase CLI