Handle missing apex_contributions

Some minimal branches have v/g_s/build/release (where apex_contibutions
have been set to mainline prebuilts), but not v/g/b (where the apex_contributions
for prebuilts have been defined). These minimal branches are unsuitable
for building a product that consume mainline prebuilts, but they would
still like to do aosp product builds.

aosp products should not use the mainline prebuilts anyways, but this
has been implemented as
- always create the dependency edge to the selected apex contributions
- do not visit the dependency edge subsequently if
  IgnoreApexContributions is set set to true

To support aosp product builds in minimal branches, this CL updates the
implementation to skip creating the dependency edge when
IgnoreApexContributions is set to true

Test: go test ./android
Change-Id: Iaa0971760e64f9b7a03542f179231ce2268b6616