ANDROID: GKI: gki_defconfig: CONFIG_DM_DEFAULT_KEY=m

Different vendors might need slightly different verions of this module
to deal with their hardware variances. So, make it a module.

Bug: 154836329
Change-Id: Iefb4ff8cf6a0123fe9af12fcfa29bc2f1eec63ef
Signed-off-by: Saravana Kannan <saravanak@google.com>