ANDROID: block: Introduce passthrough keyslot manager

The regular keyslot manager is designed for devices that have a small
number of keyslots that need to be programmed with keys ahead of time,
and bios that are sent to the device need to be tagged with a keyslot
index.

Some inline encryption hardware may not have any limitations on the
number of keyslot, and may instead allow each bio to be tagged with
a raw key, data unit number, etc. rather than a pre-programmed keyslot's
index. These devices don't need any sort of keyslot management, and it's
better for these devices not to have to allocate a regular keyslot
manager with some fixed number of keyslots. These devices can instead
set up a passthrough keyslot manager in their request queue, which
require less resources than regular keyslot managers, as they simply
do no-ops when trying to program keys into slots.

Separately, the device mapper may map over devices that have inline
encryption hardware, and it wants to pass the key along to the
underlying hardware. While the DM layer can expose inline encryption
capabilities by setting up a regular keyslot manager with some fixed
number of keyslots in the dm device's request queue, this only wastes
memory since the keys programmed into the dm device's request queue
will never be used. Instead, it's better to set up a passthrough
keyslot manager for dm devices.

Bug: 137270441
Bug: 147814592
Change-Id: I6d91e83e86a73b0d6066873c8a9117cf2c089234
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>