Limit build visibility of modules in apex/...

This changes the default visibility of soong modules in this directory
to __subpackages__, from the current public default.

This helps accidental dependencies sneaking in without intent from
the module owner.

Bug: 175604587
Test: m nothing
Exempt-From-Owner-Approval: cp
Merged-In: I1df8b8e087d27f62ce8c09d7e6e32d511efdba6b
Change-Id: I1df8b8e087d27f62ce8c09d7e6e32d511efdba6b