snapuserd: Async I/O for block verification

Boot time improvements:

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Incremental OTA of 300M between two git_master branches on Pixel 6:

Android S (with dm-snapshot):

BootComplete                  : 39.379 seconds

Android T (with io_uring):

BootComplete                  : 32.001 seconds

Time taken for each partition:

snapuserd: ReadBlockAsync complete: 2055 ms Block-device: /dev/block/dm-21 Partition-name: system_ext Size: 399302656
snapuserd: ReadBlockAsync complete: 2956 ms Block-device: /dev/block/dm-23 Partition-name: vendor Size: 650084352
snapuserd: ReadBlockAsync complete: 3534 ms Block-device: /dev/block/dm-20 Partition-name: system Size: 859746304
snapuserd: ReadBlockAsync complete: 7808 ms Block-device: /dev/block/dm-22 Partition-name: product Size: 3030687744

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Bug: 202784286
Test: Full/Incremental OTA
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I615f9f4fde4e565aa1d611a2d6bbf6a6f62fa3f1