App Pairs: App pairs now save with automatic default name and custom split ratios

[App Pairs 4/?]

This patch includes:
- Ability to save and launch app pairs with custom 0.3, 0.7, or 0.5 split ratios
- App pairs now save with default name "App1 | App2"
- Grid migration, prediction, and other misc. flows should now work with app pairs

Flag: ENABLE_APP_PAIRS (set to false)
Bug: 274189428
Test: AppPairsControllerTest.java
Change-Id: I46dd3e30f8de03dd3e6d086c8ff9fc52e7e7fd2b