Fix android_abi.gradle to use ANDROID_NDK_HOME instead of NDK_ROOT environment variable.

Author: jonsimantov

android_build_files/android_abis.gradle

@@ -12,20 +12,20 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Set the list of Android ABIs depending on whether we the NDK_ROOT environment
-// variable contains the text 'r16b' in it. This will generally only occur
-// during C++ packaging, which uses /tmp/android-ndk-r16b as the NDK directory.
-// When using this NDK version, add a few additional ABIs that are only
-// supported in r16b and earlier.
+// Set the list of Android ABIs depending on whether we the ANDROID_NDK_HOME
+// environment variable contains the text 'r16b' in it. This will generally only
+// occur during C++ packaging, which uses /tmp/android-ndk-r16b as the NDK
+// directory.  When using this NDK version, add a few additional ABIs that are
+// only supported in r16b and earlier.
 
 android {
   defaultConfig {
     ndk {
       // Default list of ABIs available in up-to-date NDK.
       abiFilters "x86", "armeabi-v7a", "arm64-v8a", "x86_64"
 
-      if (System.getenv('NDK_ROOT').contains('r16b') ||
-          System.getenv('NDK_ROOT').contains('r11c')) {
+      if (System.getenv("ANDROID_NDK_HOME").contains("r16b") ||
+          System.getenv("ANDROID_NDK_HOME").contains("r11c")) {
         // Deprecated ABIs are added to the list when building using older NDKs only.
         // Rather than an exhaustive list, we only support r11c and r16b.
         abiFilters.add("armeabi")