Set default display to be fullscreen.

Most tests written in WmTests assumes the default display is fullscreen
display, so some tests may fail when running on devices with FEATURE_PC
set and the primary display created in freeform mode. Set it to
fullscreen mode so that tests can pass on those devices.

There are some tests in WmTests for freeform display, which all run on
external displays and are not affected by this change.

Bug: 145836235
Test: Existing tests pass.
Test: atest ActivityRecordTests

Change-Id: Ibe86ae32079582aac71db123dabd0b6a9687657d