OEM-configurable max screen magnification scale

Android currently maintains a constant maximum magnification scale
of 8x for the screen magnifier accessibility feature. This leaves OEMs
with no option to provide higher magnification scales to their users.
This change allows OEMs to set their own maximum magnification
scale using a system property, with a fallback of Android’s default
8x scale.

Test: manual
Steps:
1. Set the system property `ro.config.max_magnification_scale` to a
numerical value
2. Build the images and flash to device
3. Enter Settings > Accessibility > Magnification and toggle
“Magnification shortcut” on
4. Tap on the magnification shortcut
5. Zoom in with hand gestures on the full screen to reach maximum
magnification
6. Ensure that the maximum magnification attained is visually scaled
to the value set as compared to the standard Android 8.0

Change-Id: I703f1be3d1e8b5e25d6e85b60ef4778c2898ea6c