fastboot: Use asynchronous operations to send data

The fastboot command currently uses USBDEVFS_BULK to transfer data
(including image data) to the target. On the kernel side it looks
like this:

1. Allocate a contiguous memory region and copy the user data into
   the memory region (which may involve accessing storage).
2. Instruct the driver to start a DMA operation.
3. Wait for the DMA to finish.

This is suboptimal because it misses out on a pipelining
opportunity. We could be doing 3 for the current operation in parallel
with 1 for the next operation, so that the next DMA is ready to go
as soon as the current DMA finishes.

The kernel supports asynchronous operations on usbdevfs file
descriptors (USBDEVFS_SUBMITURB and USBDEVFS_REAPURB), so we can
implement this like so:

1. Submit URB 0
2. Submit URB 1
3. Wait for URB 0
4. Submit URB 2
5. Wait for URB 1
and so on.

That is what this CL implements. On my machine it increases transfer
speed from 125 MB/s to 160 MB/s using a USB 3.0 connection to the
target (Pixel 8).

Bug: 324107907
Change-Id: I20db7ea14af85db48f6494091c8279ef7a21033d