DO NOT MERGE Watchface sample: Accessibility: Fixed "unlabeled button" on linear navigation.

bug: 37351612

Test: manual:
1. Enable Talkback
2. Long-press on watchface to enter config activity.
3. Swipe left to select each complication button. It should announce what's in the slot,
or say "Add complication". Verify it does NOT say "unlabled button."

Change-Id: Iff6b7477f8e83ce07e13338b15c8e8b4a165f0dd
Merged-In: Iff6b7477f8e83ce07e13338b15c8e8b4a165f0dd