Get max-download-size from device during fastbootd for flashall/update

Currently, during a 'fastboot flashall/fastboot update', the 'getvar
max-download-size' command is issued once to the device when it is in
bootloader mode and the same value is used even after the device boots
into fastbootd. If the max-download-size returned by bootloader is
greater than the max-download-size in fastbootd, this could break flash
as large images are broken down into chunks before downloading by using
the max-download-size variable. This will cause fastbootd to return
an error since it checks whether the buffer being downloaded has a size
greater than the max-download-size limit.

Test: fastboot flashall
Bug: 536870912

Change-Id: Ife7c1ec0583d80d4a31ecf01f1fc14a8365afe0d