Support to start tasks into split screen with specific split ratio

Split tasks can be shawn with different split ratio in overview panel.
Supports to specify split ratio when launching split tasks from
overview.

Fix: 207479945
Test: atest WMShellUnitTests
Test: Quickswitch back to app pair keep split ratio
Change-Id: Ifa6d57c0ec9b75fbcaeb80a11a95f65af8c09a0b