By default, tap gesture posture mapping should be empty

If the posture mapping is empty, AmbientDisplayConfiguration
will instead use the default tap gesture config. Before,
it was filled with empty items which caused the tap gesture
not to appear in Settings.

Test: manual
Fixes: 203608945
Change-Id: I858f63a4a949002926d20a331b2b41b3bb50a6a5