Don't use initializer_list as return type

The underlying array may be cleaned up once its lifetime has ended,
the initializer_list would become ill-formed. Return as std::vector
instead.

This fixes "-Wreturn-stack-address" (clang) / "-Winit-list-lifetime"
(gcc) warning.

Test: mma
Bug: 111998531
Change-Id: Ie5bb6bc3d0d7689744fd573c5683b22e6fb6b178