Refactor immune region logic into its own file.

Added immune_region.cc/.h in the collector directory. Changed the
functionality to no longer assume spaces are added to immune region
in ascending order.

Change-Id: Id1d643b3849ad2695e8a151dbbb74a5035644472
diff --git a/runtime/Android.mk b/runtime/Android.mk
index 98bec85..03439b6 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -52,6 +52,7 @@
 	gc/accounting/mod_union_table.cc \
 	gc/accounting/space_bitmap.cc \
 	gc/collector/garbage_collector.cc \
+	gc/collector/immune_region.cc \
 	gc/collector/mark_sweep.cc \
 	gc/collector/partial_mark_sweep.cc \
 	gc/collector/semi_space.cc \