Tweak MTP startup routine

* As a precaution, (non-permanently) disable MTP if TWRP crashes.
* Remove a duplicate logging of 'Starting MTP'.
* Combine and improve readability of conditions before MTP starts.
* Remove unnecessary Disable_MTP() call if conditions unsatisfied.

Change-Id: I61ac772fc123da5b84c3061fbcf2116531bc4e2f