AndroidRuntime: Add flag to support tombstoned aided stack dumps.

We introduce a new property dalvik.vm.stack-trace-dir that indicates
that we're using one trace file per dump. The runtime does not need
to know what the directory is, since it will communicate with
tombstoned in order to obtain a file descriptor to which it will write
its traces. In this change, we set the -Xtombstonedtraces flag whenever
dalvik.vm.stack-trace-dir is set.

Test: Manual
Bug: 32064548
Change-Id: I05984e91658250ad8379b821ca31649f13868580