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
diff --git a/partitionmanager.cpp b/partitionmanager.cpp
index ea8693f..908730e 100644
--- a/partitionmanager.cpp
+++ b/partitionmanager.cpp
@@ -2081,8 +2081,6 @@
 		gui_err("mtp_already_enabled=MTP already enabled");
 		return true;
 	}
-	//Launch MTP Responder
-	LOGINFO("Starting MTP\n");
 
 	int mtppipe[2];
 
@@ -2105,7 +2103,7 @@
 		TWFunc::write_file("/sys/class/android_usb/android0/idProduct", productstr);
 		property_set("sys.usb.config", "mtp,adb");
 	}
-	/* To enable MTP debug, use the twrp command line feature to
+	/* To enable MTP debug, use the twrp command line feature:
 	 * twrp set tw_mtp_debug 1
 	 */
 	twrpMtp *mtp = new twrpMtp(DataManager::GetIntValue("tw_mtp_debug"));