ART: Fix exception stash in stack dump

It is the executing thread that needs its exception stashed. This
may be different from the dumped thread, which, in case it is not
the same thread, the verifier does not care about.

Use RAII for stashing. Make the code simpler by removing the flag
and instead use a non-null exception object as flag.

Bug: 133764636
Test: m test-art-host
Change-Id: I2b6a44f41880c96a0cf9bb343db22963dca0a758
1 file changed