Firestore: Fix global refs exhaustion issue in Android (#1364)

Author: dconeybe

firestore/CMakeLists.txt

@@ -152,6 +152,8 @@ set(android_SRCS
     src/jni/array.h
     src/jni/array_list.cc
     src/jni/array_list.h
+    src/jni/arena_ref.cc
+    src/jni/arena_ref.h
     src/jni/boolean.cc
     src/jni/boolean.h
     src/jni/call_traits.h

firestore/integration_test_internal/CMakeLists.txt

@@ -125,6 +125,7 @@ set(FIREBASE_INTEGRATION_TEST_PORTABLE_TEST_SRCS
 
 # These sources contain the actual tests that run on Android only.
 set(FIREBASE_INTEGRATION_TEST_ANDROID_TEST_SRCS
+    src/android/arena_ref_android_test.cc
     src/android/field_path_portable_test.cc
     src/android/firestore_integration_test_android_test.cc
     src/android/geo_point_android_test.cc