MTP: Build flag for setting custom MTP device/path

Change-Id: Ic19ec61dc6cb08df00eb1326d96262b46bb93bfb
Fix handling of MTP fork/child process

Change-Id: Iacd8a38d21e9c8b0dfafefbeb646f46cd3ba02a1
Merge remote-tracking branch 'upstream/android-4.4' into android-4.4