Watchface: made complication config activity accessible.

bug: b/37351612

test: manual:
1. Enable Talkback
2. Long-press on watchface to enter config activity.
3. Tap once on complication slots. It should announce what's in the slot,
or say "Add complication".

Change-Id: Ic6f6abfe74c1d280be5a49dec5b8f0d9f7b84aef
(cherry picked from commit 8499f7b51752f3a4d0362bffa652f2c57fad86bb)