Revert "Add scan background mode to Settings.Global."

This reverts commit bf9f70e1.

Reason for revert: aosp is automatically merged to the internal tree. Since this was submitted outside the tree too, it's causing problems it seems. https://android-build.googleplex.com/builds/branches/git_mirror-aosp-master-with-vendor/grid?head=4659773&tail=4659610

Change-Id: I086d636ce8c7b67b33904195e6977e6c333e08ba