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This adds a Swap File interface which is a generic interface
which contains operations to write to, read from, and drop
contents from a swap file. We also add three different swap
file implementations on top of the standard swap file, which
include a compressed, encrypted, and compressed + encrypted swap
file.

The compressed swap file implementation is a standard gzip
implementation. The encrypted swap file uses AES 256 GCM SIV
mode with a 256bit ephemeral key which is randomly generated
and a randomly generated nonce. All swap files will write to
/mnt/stateful_partition/unencrypted/swap if it exists otherwise
swap file creation will fail. The key never leaves the internal
EncryptedSwapFile implementation.

The swap files are created as O_TMPFILE | O_EXCL meaning they
do not actually link and O_EXCL prevents them being linked to
the file system, so all space is reclaimed when the file
descriptor is closed.

Disk space can be incrementally reclaimed after memory has
been read out of the swap file using the DropFromSwap API.
It uses fallocate(2) with FALLOC_FL_PUNCH_HOLE.

Bug: 1067833
Change-Id: Ifeacec0ed5571507f80add2cf43fdd4063478459
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2142474Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Brian Geffon <bgeffon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759044}