ANDROID: gki_defconfig: enable dm-default-key

dm-default-key is needed for metadata encryption
(https://source.android.com/security/encryption/metadata

).

The new version of dm-default-key is vendor-independent and can be used
both with and without inline encryption hardware.

Bug: 137270441
Bug: 147814592
Change-Id: I92ce45c4b1543ff8bc5cdb5ebe4ddcdb740c2dc7
Signed-off-by: Eric Biggers <ebiggers@google.com>