Load vibration scale gain from system properties

Update external vibration scaling library to load the scale gain used to
define the scale factor for each level (low, medium, high).

This allows vendor to configure the scale levels, similar to how they
can also configure the haptic generator output gain.

Bug: 356407380
Test: libvibrator_test
Flag: android.os.vibrator.haptics_scale_v2_enabled
Change-Id: I67753c32c00f706bbbdf50f999c222fb9ade4224