Make ruleToGenerateHiddenAPIStubFlagsFile build rule

Previously, the func created a rule and returned it for the caller to
create with the appropriate name and description. This change passes
the name and description into the func and causes it to create the rule
itself. The func is also renamed to make it more consistent with the
other similar rules.

Bug: 179354495
Test: m nothing
Change-Id: I2a4455daa8a6090ed5568994b255848d063e1ab2