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

(cherry picked from commit 8d1594da6e97cd11580baf30ee4b75d4e7e2616a)

Change-Id: Iad3f3dde63155993bb4efb7bbbbde3e36f1379cc
1 file changed