ART: Allow unwinding unattached threads

Partial revert of commit ed8b723c5f3989d2593ec21c65c96d6d8bf25579.

Make it (constexpr) configurable whether we allow unwinding native
stacks of unattached threads.

Bug: 27449879

Change-Id: I307a80c9a0166f33fa8a41e492d7eb0d77d0e44f
1 file changed