mirror-repos.py: Process submodules from multiple train branches

Renamed get_latest_train_branch to get_latest_train_branches and made it return the two latest train branches, being sure to only match proper train version numbers.

get_submodule_urls: Allow branch to be a list of branches and collect the unique set of submodule urls found across all the specified branches. This helps for cases where submodules move from gerrit to gitlab or when a submodule is added/removed from production.

Change-Id: Ibd94c2bcf0f5581c5ef9938874d5a8d8b05b33bb