Added getFiles method to DeviceFolderBuildInfo

Base BuildInfo class is missing device images.

Test: Unit test on new method.
Bug: b/62223650, b/30815281
Change-Id: Ib63affc488f7c3f231083470da33ff9f25abb998
(cherry picked from commit 8f92eff05258604b253719aa7892079911261890)
3 files changed