Make export_includes a restat rule

There is currently an intentional incremental rebuild issue with
import_includes.  export_includes might get updated with an identical
version, but we don't want to force everything downstream of it to
rebuild.

When BUILDING_WITH_NINJA==true, only update export_includes if it
changes, and use .KATI_RESTAT to only run downstream rules if it
changes.  import_includes will only be updated if one of the
export_includes files is updated, so object files can have a normal
dependency on import_includes instead of an order-only dependency.
All downstream object files will now be recompiled if their imported
include paths change.

Bug: 25910568
Change-Id: I626f3b24ac02ac1309049cf1ce66cfe8ec816513