Ensure that adbd is stopped explicitly before re-enabling it

Simply putting the Global setting and waiting for the ContentObserver
created a race condition, where reading the updated setting may skip
reading the value that triggered the change. In this case, it meant that
we never toggled off ADB. This explicitly disables and kills ADB before
re-enabling it.

Bug: 150721793
Test: Manually, running adb shell cmd testharness enable on user and
userdebug builds

Change-Id: Ic2b3aadbadc2bfe52440066aa98f9a2238ae3bb9