Remove Object* weak roots from the debugger.

The weak roots were converted to JNI weak refs.

Since the weak roots are now normal JNI weak refs, we eliminate the
need to insert read barriers for those weak roots in the debugger and
the need for the GC to have a separate step to update them as part of
the system weak sweeping.

Bug: 12687968
Change-Id: If16396d4713457b8af4f1ea6a0c6ec6799cb615e