Created 4 camera preview stress test cases.

The 4 stress test cases cover situations of preview switch between
front/back camera and between photo/video mode. Followed Google
Camera App Kenai version streaming rules to set preview request in
the tests.

b/28916141 will be solved once this change is merged

Change-Id: I448be429df5b0813fa2e2f6ff05ad2936afee1b3