Only change USB mode to mtp when ready for MTP

Commit 'Do not toggle USB ID during MTP startup if not needed' changed
the default USB mode to 'mtp,adb'. Depending on the device, this can
result in an unfortunate side effect of crashing an ORS sideload
midway through the process (it's not clear to me whether the kernel or
the connected computer is responsible). Only put USB into mtp mode
when MTP storage is starting-up.

The hack to change the USB mode to adb when a user compiles TWRP
without MTP support is no longer necessary now that 'adb' is the
startup mode.

Change-Id: I8ed13d6ab8e85621533997b8c37ef7ebec0fcf85