Split out Dalvik implementation

This helps with compiling this on non-Android platforms since you can
simply not compile the Dalvik implementation.

Test: mmma -j32 libcore && vogar --mode host --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-support-hostdex_intermediates/classes.jack --classpath out/host/common/obj/JAVA_LIBRARIES/core-tests-hostdex_intermediates/classes.jack libcore/luni/src/test/java/libcore/java/lang/OldClassTest.java
Change-Id: I44ae710f36a4ce44d8c3615e371522c6b56c8e1d