Switch from core to core-libart

Bug: 14298175
Change-Id: I1db40e7c67322d80a108b2b88e6d2e6d275d7898

Conflicts:
	core/base_rules.mk
	core/droiddoc.mk
	core/host_dalvik_java_library.mk
	core/host_dalvik_static_java_library.mk
	target/product/core_base.mk
	target/product/core_minimal.mk