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

Bug: http://b/26403620
(cherry picked from commit 0359d778a564a5c63baba08a33484fb69eda4d1e)
