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

firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk,firebase/firebase-cpp-sdk