Add One-Handed mode features

1. Settings keys definition.
2. Support Backup/Restore for user settings.

Add Settings keys definition for toggles.

Toggles in Settings location:
Settings > System > Gesture > One-Handed

Bug: 146908717
Bug: 151130376

Test: manual
Change-Id: Ia329e39a7f1912620ba8ba5b47619236fb872509