Allow devices to calculate their own split ratio instead of using fixed ratio

Previously, the 30-70 and 70-30 ratios were always calculated precisely at 34.15%, defined in docked_stack_divider_fixed_ratio.

This CL adds a boolean flag, config_flexibleSplitRatios, that allows devices to override docked_stack_divider_fixed_ratio. When overridden, the device will possibly use a larger split ratio to ensure that apps are shown with a minimum size.

Fixes: 330388049
Test: Manual
Change-Id: I491e895abab075e0319804fdec21f5e94053e7f3