fix(magnifier): fix deadlock when feature flag changes

Handle feature flag changes on a background thread so we're not blocking
the main thread which might already be locked.

Fix: 281132229
Test: 0. Complete initial SUW without connecting WiFi, no account added and no lock screen set.
1. On target device, tap on deferred SUW notification and proceed.
2. Tap on "Get started" on Welcome screen. Quick Start User Guide screen is displayed.
3. Connect source and target devices by scanning the QR Code with scanner
4. WiFi setting is transferred and the target device connected to internet. Getting your phone ready screen is shown
6. Wait and observe. SysUi should not hang / black screen.

Change-Id: Ic2cf5cfb34041671468f0036a243e24005d064c9