Preserve or swap split-screen layout ratio on app drop

Maintain split-screen layout ratio (0:50, 30:70, 70:30, 90:10) when an app is dropped into an existing split-screen configuration. For 10:90 layouts, dropping an app into the 10% side will swap the ratio (e.g., 90:10 becomes 10:90).

Calculates and passes the current split-screen ratio to the split transition. Introduces `mIsDropVisibleSplitEntering` to distinguish from existing split entry logic.

Bug: 424694807
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Manual
Change-Id: Ifb1c44afa2a5b137de0960e1e02fa93d6610a9c6