Add setting to disable animations

Fixes: 12910879
Test: make RunSettingsRoboTests
Also went to Settings -> Accessibility, turned off animations,
and observed that animations stopped. Turned it back on, and
the came back. Also fiddled with the granular settings in
developer options, which behaved as expected.

Change-Id: I189fbc4152d3cdb5ee45562c33f94c739f569c32