ensure /data/tombstones exists on all Android devices

/data/tombstones is referenced by core platform code, but is not
guaranteed to exist on all Android devices. Move the directory
creation out of device specific files and into the core
init.rc file.

Bug: https://code.google.com/p/android/issues/detail?id=93207
Change-Id: I94ae5199a6a32c4fe555ca994fc4a8345e0c9690