Keep pointer to original DexFile during JVMTI redefine for hiddenapi

JVMTI redefine overwrites the pointer to the class' DexFile which
prevents access checks from reading the hiddenapi flags store. Store
the pointer in ClassExt together with the original ClassDef index
to preserve the access to flags store. Because method/field indices
are still lost, the corresponding dex member is found using string
comparison of member's name and type.

Bug: 119688837
Test: 999-redefine-hiddenapi
Change-Id: Ifdf35668e838869a971233bbaae61851014658b1