Unversion NDK stub libraries.

The system versions of these libraries aren't versioned yet.

Bug: https://github.com/android-ndk/ndk/issues/278
Test: make ndk
Change-Id: I74e988be8f60f8728a0bfa7360091cbd9f5b1186

native/android/Android.bp

@@ -17,4 +17,5 @@ ndk_library {
     name: "libandroid.ndk",
     symbol_file: "libandroid.map.txt",
     first_version: "9",
+    unversioned_until: "current",
 }

native/graphics/jni/Android.bp

@@ -17,4 +17,5 @@ ndk_library {
     name: "libjnigraphics.ndk",
     symbol_file: "libjnigraphics.map.txt",
     first_version: "9",
+    unversioned_until: "current",
 }