adb: Use kernel aio for functionfs.

This method works around the downsides of
ENDPOINT_ALLOC, namely that it is not affected
by memory fragmentation and it uses an upstream
interface.

Also add libasyncio to provide the necessary syscalls
to both adb and mtp.

Add some small optimizations to file_sync.

Bug: 37916658
Test: run adb push/pull
Change-Id: If3b3be02b5e2d4f9cffec1b8ddc02a5768a51a1f