Move logic for selecting the primary module into initHiddenAPI()

Bug: 178361284
Test: m droid
      Verified that hiddenapi files (both aggregated ones and for the
      individual modules) are not affected by this change.
      Also verified that the hiddenapi files created when using the
      prebuilts (using SOONG_CONFIG_art_module_source_build=false) are
      the same as when using the source. There is a slight difference
      in the order but otherwise identical.
Change-Id: I831da5fa41eb7668b07bbdc4dd08ec2cdb8803d0