Consolidate last reported windowing mode of activity

The lifetime of the windowing mode should be the same as
ActivityClientRecord, so it can be a field of the class.
Then there is no need to manage an additional container
for mapping the token and windowing mode.

Also remove some unused fields.

Bug: 163976519
Test: CtsWindowManagerDeviceTestCases
Change-Id: Ic3d9b5450479de8fcaea1f5b608a3e3132e34781