Move sleep during MTP startup to MTP thread

Some devices are very slow to respond to the sysfs requests. To
prevent delaying the main GUI from booting during TWRP startup, we
move the sleep delay to just before we open the MTP device and
into the MTP thread so that it does not hold up the main TWRP
thread.

Change-Id: Ic931ef317d0fb7ef4dfdef46a32f68a014ff62c0