Start DisplayRotationCompatPolicy upon creation.

Also add tests for `start()`: these policies are created and started
 in the `DisplayContent` constructor, thus we cannot spy on them
and `verify` that `start()` methods are called.

Test: atest WmTest:AppCompatCameraPolicyTest
Bug: 359099136
Flag: EXEMPT bug fix
Change-Id: Ibc9f808dbf76fa1b2e4dafe8ca48a8a57baca3df