Supply apex_info.pb as parameter to delta_generator if file exists

Supply generator with parameter --apex_info_file=META/apex_info.pb
if the file exists.

This ensures that apex_info is included in payload header.

This is identical to the behaviour of brillo_update_payload
which is not being used since:
Invoke delta_generator directly
fcd731e3

Issue: 286253576
Test: Manual, confirm that apex_info is included in payload header
Change-Id: Ic096c5f8966beec8686f918aba462c955290a6c5