Fix build failure when building unbundled apps (second try)

The previous attempt, which simply skipped the hidden API processing
altogether when unbundled builds were enabled failed when attempting to
build module snapshots as while they enabled an unbundled build they
actually need the hidden API processing to be performed.

This change just checks whether missing dependencies are allowed and if
so it fakes up any missing files so that the build will only fail if
they are not present AND they are used.

Bug: 186695448
Bug: 185828824
Test: tapas Calendar
      m -j60

Change-Id: Ie13fed05af0aba51f45f6791fce944d0e4285037