Enlarge USB bulk transfer size for faster downloads

The default USB transfer bulk is fixed as 4096 in fastboot util code for
Windows and Linux. Enlarging the bulk size can greatly improve the image
download speed via USB.

For Windows, adjust the max bulk size to 1MB to maximize the USB transfer
speed. With this change, the USB transfer speed can be doubled to 20MB/s.

For Linux, adjust the max bulk size to 16384 to maximize the USB transfer
speed according to MAX_USBFS_BUFFER_SIZE definition in drivers/usb/core/devio.c.

For OSX, the maxLenToSend is already 1MB in code.

Change-Id: If6af8c6301f6f6c2ef345e37241706f16d8f5cda