android_build_files/android_abis.gradle: Verify that System.getenv("ANDROID_NDK_HOME") is not null prior to using it.

This fixes the following cryptic build error if the "ANDROID_NDK_HOME" environment variable is not set:

java.lang.NullPointerException: Cannot invoke method contains() on null object

Author: dconeybe

android_build_files/android_abis.gradle

@@ -24,8 +24,9 @@ android {
       // Default list of ABIs available in up-to-date NDK.
       abiFilters "x86", "armeabi-v7a", "arm64-v8a", "x86_64"
 
-      if (System.getenv("ANDROID_NDK_HOME").contains("r16b") ||
-          System.getenv("ANDROID_NDK_HOME").contains("r11c")) {
+      if (System.getenv("ANDROID_NDK_HOME") != null && (
+              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")