Make SmartReplyConstants thread-safe.

To be able to move smart reply view inflations off the UI thread we need
to access SmartReplyConstants on a background thread. Thus, in this CL we
make SmartReplyConstants thread-safe.

Bug: 119801785
Test: none
Change-Id: Ib62fc078fcb05ec4795a29836d5b10443516ca8e
1 file changed