build: Update gradle to allow enabling debug mode of NDK builds

When NDK_DEBUG environment variable is set, it is passed to ndk-build,
which will build in debug mode and include appropriate version of gdbserver.

Change-Id: Ife8e4c6cd798f2dadcc4c00fc3b5f39dacb2e516
diff --git a/.gitignore b/.gitignore
index 7ed56cf..eb380e5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -12,6 +12,7 @@
 bin/
 gen/
 out/
+obj/
 build/
 app/src/*/libs/
 app/src/*/obj/