Adds a new config flag, config_magnification_area

We adds a new config flag to control whether to enable
"magnification area" of the window magnifier.
If "magnification area" is not enabled, we would hide
the settings entry in magnification settings
and adjust both the magnifcation
capability settings and the promt settings.

Bug: 177371954
Test: none
Change-Id: Ib14dc6f48d5695c5729b2552cac72ac603db0436