aconfig: add default-permission argument for create-cache command

Aconfig set flag default permission as READ_WRITE regardless of the
release configuration. This change enables the caller of create-cache to
set the default permission for all flags, if the flag doesn't have a
value.

Test: atest aconfig.test
Bug: 294417368
Change-Id: I1ba19e1ba793cd2ae59923b136b9b50a92315ece