init: don't start tombstoned until /data is mounted.

When vold mounts the encrypted /data partition, it first checks for and
kills processes that have open fds to the tmpfs placeholder at /data.
This resulted in a 20 second boot-time regression (vold's timeout period)
when tombstoned was started before vold.

Bug: http://b/34461270
Test: boot is faster, no messages from vold in console spew
Test: tombstoned still started by init
Change-Id: Ib5e9ddb05f40c9da852f00e103861c6ff2d94888