Add custom finishing behaviors for split pair

Add support for custom finishing behaviors for containers in split
pairs. 'finishPrimaryWithSecondary' and 'finishSecondaryWithPrimary'
now have several options:
- never
- always
- adjacent (finish only when containers are shown side by side, but
not when stacked).

'finishPrimaryWithSecondary' option was also added to placeholder
rules.

Bug: 204829538
Test: Manual, using demo app and Settings.
Change-Id: I543d15e62acc57f9ad6578221248fbb0a8c719db