Android.mk: add a check for NDK_ROOT

This simplifies integration with the Android platform and avoids the
files from being used when a non-NDK build is performed. In that case
Android.bp is preferred.

Change-Id: I027ded298ae518db5388a7e514f2ef7b1d940b4b

Android.mk

@@ -1,3 +1,5 @@
+# Ignore this file during non-NDK builds.
+ifdef NDK_ROOT
 LOCAL_PATH:= $(call my-dir)
 
 include $(CLEAR_VARS)
@@ -18,3 +20,4 @@ LOCAL_LICENSE_KINDS := SPDX-license-identifier-BSD
 LOCAL_LICENSE_CONDITIONS := notice
 LOCAL_NOTICE_FILE := $(LOCAL_PATH)/LICENSE.TXT $(LOCAL_PATH)/PATENTS.TXT
 include $(BUILD_STATIC_LIBRARY)
+endif  # NDK_ROOT