Add and update cross device sync data holders

This allows easier handling of the sync messages as they are
prepared for transmssion across the CDM channel, and for processing
once they arrive.

Bug: 265466098
Test: atest FrameworksServicesTests:com.android.server.companion.datatransfer.contextsync
Change-Id: I597899971bb24b9318b99ed8f5b1625eaef6c9b8