Make mirror window accessible (2/2)

To support accessibility, we added a11y actions to move
the mirror window and change its scale. We also provide
content description, state description to deliver the
information of the mirror window.

Finally, we update state desceription to send window content change
 when changing the scale.

Test: manually test with voice access
      atest WindowMagnificationControllerTest
Bug: 143852371
Change-Id: Ia27c5bee39e732609ac1d8e2c7566524e6d7c64c