Add more read barriers for JNI roots.

To make it possible to concurrently scan the JNI global roots (that
is, the roots visited by JavaVMExt::VisitRoots()), add read barriers
to the indirect reference table and the reference table.

Also, add read barriers to the jmethodID/jfieldID decode routines
(ScopedObjectAccessAlreadyRunnable::DecodeField/DecodeMethod) so that
we can concurrently handle (encoded) fields and methods.

Bug: 12687968
Change-Id: I3df4e4e622a572ff0ea8d44b2dc70a4d6b3ba058