ART: Refactor UnstartedRuntime for testing

Expose the UnstartedRuntime implementation functions as private static
methods of a class. Add a gtest that can invoke these functions. Add
sample tests for String and Memory.

Bug: 21173514

(cherry picked from commit 799681b1)

Change-Id: Ib5bde6347fafaf7607c642542ea7d5938ff4b1df