Add prebuilt_visibility property

Some mainline modules provide an `sdk` and one or more set of
module_exports. When the snapshots are unpacked into separate prebuilts
directories (one per snapshot) then any dependencies from a member of
one snapshot to a member of another may require the latter to have
additional visibility rules.

Previously, these rules had to be added to each source module. This
change allows additional visibility rules to be specified on the
sdk/module_exports that are added to all the prebuilts in its snapshot.

Bug: 155921753
Bug: 168301990
Test: m nothing
Change-Id: Ia3eacb5749981f04770ae9872a8013e43c5c6ef1