Introduce flat deps info list.

Compared to full list, flat list drops dependency edges and simply
lists all transitive dependencies for a top-level apex bundle.

Bug: 149622332
Test: m, manually build flatlist
Change-Id: Ibd521c96b7aeab90b95965c1b524e0a0152aaf5a
Merged-In: Ibd521c96b7aeab90b95965c1b524e0a0152aaf5a
Exempt-From-Owner-Approval: cp from aosp
(cherry picked from commit a8bd113a)