feat(#AlwaysOnMagnifier)!: Support always on feature in FullScreenMagnificationController

Add variable mAlwaysOnMagnificationEnabled in FullScreenMagnificationController.
If enabled is true, when onUserContextChanged method is triggered, the magnifier will zoom to 100% and keep in activated state.
If enabled is false, the magnifier will be disabled. It's the original behavior.

Bug: 146504200
Test: manual
      atest FullScreenMagnificationControllerTest
Change-Id: Id7be1fe2cb66ce079f413103ea4e134b4542571c