Update util method to detect touch event support

Before we would 'cache' the result but only not recalculate it if it was truthy. Now we check for undefined so return cache as long as it's not undefined.

@bug W-2050623@
@rev amcculloch@
Fix failing unit test after addition of *.auradoc files

We have a test that looks in the def registries for components and is expecting a certain number back. The type 'auradoc' is also found as markup so adjust test accordingly.
Merging origin/tbliss/master-pending-supportsTouchEvents-cache into master