Add internal JRE support for dalvik.system.VMRuntime to support additional unit tests.

Several libcore unit tests check for the Android API level to test new features using dalvik.system.VMRuntime. We should add a limited, internal-only implementation of VMRuntime, so that when jre_emul is updated to support a new API level, additional version-specific tests will be run. This would eliminate the need to comment-out and then uncomment tests for that new functionality.