Add TestApi to always select the app requested display mode

The added test API is used to ignore all votes in
DisplayModeDirector app votes. This way only the app
requested display mode will be picked. The API will be
used for testing app mode switch requests.

Bug: 159113268
Bug: 158316271
Test: m
Change-Id: I257636ae2a6ed74044e71e49a99024682c3a92c5