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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 0c540aad)

Bug: 8122243
Change-Id: Ie97f6df1ba141b1ed8007413ec1a834b0486cc2a