Add MAINLINE_SEPOLICY_DEV_CERTIFICATES to keys.conf

DEFAULT_SYSTEM_DEV_CERTIFICATE is not appropriate as some OEMs may need
to change only the certificates used to generate
plat_mac_permissions.xml for mainline modules.

Test: m, checked output plat_mac_permissions.xml
Bug: 138097611
Bug: 134995443
Change-Id: Ie19130a243db043f432039c54c379f06e60ab6c6
2 files changed