vold: Support external non-emulated storage

It tries to acess EMULATED_STORAGE_SOURCE which is not
defined if the device doesnt have an emulated storage

mkdirs() also needs to be valid for EXTERNAL_STORAGE

See also https://github.com/CyanogenMod/android_system_vold/commit/5ced6afb769f09ada6e3d1787a36c6f9f8469ed6

Change-Id: I6463979a06eab301acd59e815713abc3f550853f