Support individual magnification scale for each display

Currently we only have a globale magnification scale which
is stored in settings. It ends up the scale is shared between
multi-displays. To provide better experience, we store the scale
for each none-default display until the device reboot,
and the default scale is 2.

Test: atest com.android.server.accessibility.magnification
      atest FullScreenMagnificationGestureHandlerTest
Bug: 194667380
Change-Id: I0638fe90ef262d287cec81919baabdf215bb2e48