fs/StandardDirectory: add GetUserCacheDir()

Move code from CreateConfiguredDatabase() and add XDG support.  This
implements an automatic Linux fallback for the setting "db_file" if
none was specified.

src/fs/StandardDirectory.cxx

@@ -50,6 +50,8 @@
 #ifdef ANDROID
 #include "java/Global.hxx"
 #include "android/Environment.hxx"
+#include "android/Context.hxx"
+#include "Main.hxx"
 #endif
 
 #ifndef WIN32
@@ -252,6 +254,18 @@ AllocatedPath GetUserMusicDir()
 #endif
 }
 
+AllocatedPath
+GetUserCacheDir()
+{
+#ifdef USE_XDG
+	return GetUserDir("XDG_CACHE_DIR");
+#elif defined(ANDROID)
+	return context->GetCacheDir(Java::GetEnv());
+#else
+	return AllocatedPath::Null();
+#endif
+}
+
 #ifdef WIN32
 
 AllocatedPath GetSystemConfigDir()