Remove -allowaccessmodification from the basic Proguard flags

It seems with that flag Proguard modifies the class files in a way that
Dalvik does not like.
Bug: 10307364

Change-Id: If0cc7552da00ae8a3a3a39a9f2f029819ca3164a
diff --git a/core/proguard_basic_keeps.flags b/core/proguard_basic_keeps.flags
index 0ce81ef..af0e05d 100644
--- a/core/proguard_basic_keeps.flags
+++ b/core/proguard_basic_keeps.flags
@@ -1,7 +1,6 @@
 # see http://sourceforge.net/tracker/?func=detail&aid=2787465&group_id=54750&atid=474707
 -optimizations !code/simplification/arithmetic
 -optimizations !code/simplification/cast
--allowaccessmodification
 
 # To prevent name conflict in incremental obfuscation.
 -useuniqueclassmembernames