Fix missing shader cache path

gyp/mbgl-android.gypi

@@ -31,6 +31,7 @@
       },
       'sources': [
         '../platform/android/cache_database_data.cpp',
+        '../platform/android/shader_cache_data.cpp',
         '../platform/android/log_android.cpp',
         '../platform/android/asset_request_baton_libzip.cpp',
         '../platform/default/string_stdlib.cpp',

platform/android/shader_cache_data.cpp

@@ -0,0 +1,13 @@
+#include <mbgl/platform/platform.hpp>
+#include <mbgl/android/jni.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default shader cache on this system.
+std::string defaultShaderCache() {
+    return mbgl::android::cachePath + "/mbgl-shader-cache-";
+}
+
+}
+}