Allow neverallow rules to be defined by custom bootstrap_go_packages

* Adds a Rule interface to hide the rule struct and expose the mutator
functions.
* Makes the neverallow() function public as NeverAllow().
* Adds AddNeverAllowRules func to add more rules.
* Moves the population of the builtin rules to init() function.

Bug: 136159343
Test: m droid
      - changed a built in rule to cause build failure to ensure the
	    rules were still being applied, then reverted the change and
		ran 'm droid' again.

Change-Id: Ie3a4456d1f6bc12c5b4931bf698333275347fdf0
(cherry picked from commit 18af0908)