adbd: limit the sizes of aio writes.

The kernel does a kmalloc of the write size for each write during
io_submit, which can lead to failure if the kernel's heap is fragmented
such that it can't allocate a contiguous 1MB chunk. Partition writes
into 16 page chunks which are much more likely to succeed.

Bug: http://b/126582877
Test: test_device.py
Change-Id: Ib2d48561594e1d81f1a2f62e34eaa40265ac47c4