Correctly init code cache path for RS

Bug: 27439261

Script Group needs to know the code cache path before it can call bcc to
merge kernels. However, before this change, the code cache path has been
initialized by the ScriptC class.

In the case where a script group (or even the entire app) does not contain any
regular script but only intrinsics, the code cache would remain uninitialized.

Fixed this by initializing the code cache path in the RenderScript class
the first time when the accessor method is called.

Change-Id: I87f9e62e0f3b479f94e43daa3e9695a5b38710db