Update and improve android_gradle (#965)

* Update CMakeLists.txt files so we can build libamber_ndk.so for
Android via CMake.

* `android_gradle/`:

  * Update Gradle version, settings, and build files to match the latest
  Android sample. Importantly, this removes the use of jcenter and
  bintray repositories, which were shut down.

  * Specify NDK version so that Gradle can install required native build
  tools automatically.

  * Define externalNativeBuild so Gradle can perform the Android CMake
  build automatically.

* Replace `main` with `android_main` when building the Amber tool as a
library for Android to avoid compiler warnings, and rename other
Android-related symbols to avoid confusion.

Fixes #964

1 files changed, 1 insertions, 1 deletions

diff --git a/android_gradle/app/src/main/java/com/google/amber/Amber.java b/android_gradle/app/src/main/java/com/google/amber/Amber.java
index 8319ccf..008c528 100644
--- a/android_gradle/app/src/main/java/com/google/amber/Amber.java
+++ b/android_gradle/app/src/main/java/com/google/amber/Amber.java
@@ -19,5 +19,5 @@ public class Amber {
     System.loadLibrary("amber_ndk");
   }
 
-  public static native int androidMain(String[] args, String stdout_file, String stderr_file);
+  public static native int androidHelper(String[] args, String stdout_file, String stderr_file);
 }