Remove hiddenAPI.primary field

Now that the individual modules no longer participate in the generation
of the monolithic files it is no longer necessary to select a single
primary module to provide the information they need.

Bug: 179354495
Test: m droid
Merged-In: If09796de710927e3e3f2ccecad0b57ca5fce5dc9
Change-Id: If09796de710927e3e3f2ccecad0b57ca5fce5dc9
(cherry picked from commit 45897dd6)