Add new setting for enabling screensavers while postured.

Postured is currently defined as the device being stationary and upright at an angle of 90-120 degrees relative to the ground

Test: N/A - adding the constant
Flag: EXEMPT constant is not yet used
Bug: 383208131
Change-Id: Ie0f9d29353a9d8fc4d66e64bf471a2ae771c870e