App Pairs (behind flag): Add Overview menu item, icon, tests

This is the first of several patches implementing the App Pairs feature behind a flag.

This patch includes:
- The icon asset for the "Save app pair" menu option
- The menu option, which will appear in the Overview drop-down menu for split pairs
- Some tests ensuring that the menu item appears when appropriate

Flag: ENABLE_APP_PAIRS (set to false)
Bug: 274189428
Test: 2 tests in TaplTestsSplitscreen.java
Change-Id: I174f2c66e7eb6d83ff7fe501eb82cc48e727022c