Change Ti::Agent collection from vector to List

We want these to be in a list to avoid the need for moving them
around, which would introduce the need to perform annoying accounting.

Test: ./test.py --host -j40
Bug: 36716572
Bug: 31455788
Change-Id: Ie16523f4e7d79934fa7844fb9b9a26e125cfa9eb
7 files changed