Add tidy.mk to set up global default checks.

* Default checks include only google* minus google-readability*.
* Some google-* checks are disabled for external projects.
* Each project can add more or disable checks through LOCAL_TIDY_CHECKS.

Bug: http://b/27779618
Change-Id: I83eed73bd2bddaace565a6dde532fb2036b4fac4