Ensure prebuilt modules have same visibility as source modules

Exports visibility and package mutator registration functions so they
can be used in sdk testing. Updates sdk test to support visibility and
package modules.

Adds EffectiveVisibility(...)[]string function to make the effective
visibility rules available to sdk snapshot creation.

Extracts compositeRule.Strings() []string from compositeRule.String()
method so that it can be used by above func.

Adds visibility property to sdk snapshot and prebuilt modules along
with a test to ensure it works properly.

Adds dir parameter to CheckSnapshot so that it can check the snapshot
generated for a non-root package. That is required in order to ensure
that visibility of :__subpackages__ on a source module in package
<pkg> is resolved to an effective visibility of
//<pkg>:__subpackages__ on its corresponding prebuilt.

Test: m conscrypt-module-sdk
Bug: 143678475
Change-Id: Icaacac5b9c04726d28e6fec93e49715ac45df7f4