Set jdemeulenaere@ and nijamkin@ as OWNERS of PlatformComposeCore

I want to make sure people in our team own this and are aware of changes
made to PlatformComposeCore, given that they might impact a lot of
features at once.

Bug: 294841578
Test: N/A
Change-Id: I93ab2f2cf2e8920348f73375fe59dcb6c0a2a2b6