Fix ConcurrentModificationException in SummaryLoader.

When the dashboard summary is being initialized, it will rebuild the UI
while the summary loader tries to to go through the tiles to update the
summary. Both is being done on a separate backgroud thread, and it will
run into concurrent modification issue if the thread is being swapped
while one is looping through the list.

Instead of letting clients access the list of tiles directly, add a
getter method in DashboardCategory to get a copy of the list of tiles
for all read-only operations.

Change-Id: I479669abd8d1d0a8ee9a4113d8ad2244da56f4d8
Fixes: 69677575
Test: make RunSettingsRoboTests