Only force reset when adb is not enabled (i.e. mtp is)

Some users depended on adbd continuing to run after disconnect
in order to use scripts with nohup. This change allows this
use case to keep working since only mtp has to be force
set.

Bug: 38227228
Test: adb shell "nohup sh /sdcard/script" where script contains
"sleep 5; touch /sdcard/done" and verified file still appears even if
disconnected during sleep.
Test: am force-stop com.android.providers.media, still connects

Change-Id: I25ae2b922fa4d06109ac8cf5e43e1c47a33c46a6
(cherry picked from commit ad962a92)