Download Home : Added selection on images

Details:
1 - Changed image grid layout to show a symmetrical grid
2 - While on selection, it would show a blue circle
3 - Photos not selected will show a holo circle instead

UX : https://docs.google.com/presentation/d/14pBMf4Qrhd_a4qF6KdXJJfAZYLdmv_JN0gL8GC4SWAU/edit#slide=id.g3a0d9650d2_6_193

Bug: 863718
Change-Id: I4c6d54b9abb4056afaaf20be0c90167c889c4b02
Reviewed-on: https://chromium-review.googlesource.com/1142743
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576557}
5 files changed