ART: Relax CurrentMethodVisitor requirements on GetDexPC

In case we want to dump a Java stack after an unhandled fault, in
case we hold a thinlocked monitor, that monitor might get inflated.
That can cause an abort as we may not have enough/correct information
for the state at the bottom-most call.

Relax GetDexPc in the CurrentMethodVisitor to not abort when it cannot
find a dex pc. Instead, let the caller handle such a case. This CL
allows the locking_dex_pc_ in Monitor to be DexFile::kDexNoIndex,
which avoids the above abort.

Bug: 16352802, 16556938
Change-Id: I3adf89b2d8f018a0c3e3abdd26e542f46ee59eef