keystore: Add flag for blobs to be unencrypted

In order to let apps use keystore more productively, make the blob
encryption optional. As more hardware-assisted keystores (i.e., hardware
that has a Keymaster HAL) come around, encrypting blobs start to make
less sense since the thing it's encrypting is usually a token and not
any raw key material.

(cherry picked from commit a3788b00)

Bug: 8122243
Change-Id: Ifc1c64743651b23a4eace208ade0176af47ea989