adb: fragment host linux USB writes when needed.

We've seen USB writes failing due to inability to allocate contiguous
chunks of memory in the kernel on devices, but it looks like the same
problem can occur on the host, as well. It's a mild performance
regression (90->80 MB/s on a blueline) to split the writes always, so
attempt the full write first, and fall back to splitting it up if that
fails with ENOMEM. Once we switch over the the asynchronous transport
API, we'll be able to submit multiple writes cheaply, like on devices,
so we won't need to retry at that point.

Bug: http://b/140985544
Test: test_device.py
Change-Id: I1517c348375b829dfff6796c4e9d394802b02d5b