Revert^2 "Move SettingsProxyExt content observer registration call on bg thread."

This reverts commit bcf59292f8e8842d329c2f43451d875edd0f36c5.

Reason for revert: b/357575679 should never have been reverted
Change-Id: Ifd8a90e92389242fcf862716f3ddff2921229d6a
3 files changed