enhancement for building app with emma code coverage

* global filter to avoid applying instrumentation on emma classes
* remove local variable information at dex step instead of compile time

Change-Id: If04c27bc717f34816077a98ead9ceb0dbcbb0d2f
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 0fe3c8b..f58e3dd 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -92,6 +92,7 @@
 LOCAL_NO_EMMA_COMPILE:=
 LOCAL_PROGUARD_ENABLED:= # '',optonly,full,custom
 LOCAL_PROGUARD_FLAGS:=
+LOCAL_EMMA_COVERAGE_FILTER:=
 
 # Trim MAKEFILE_LIST so that $(call my-dir) doesn't need to
 # iterate over thousands of entries every time.