Support individual magnification mode for each display

Currently we only have a globale magnification mode which
is stored in settings. It ends up the mode is shared between
multi-displays. To provide better experience, we store the mode
for each none-default display until the device reboot,
and the default mode is full-screen.

Test: atest com.android.server.accessibility
      atest com.android.systemui.accessibility
Bug: 194667380
Change-Id: I099e1ee945938aac07cc079f68bccbc745d40b08