Fix SysUI NPE crash during the boot/init progress

When Device boot and SystemUI servies starting, settings provider
may callback onChange() when OneHandedController register observer.
If the callback timing earlier han mEventCallback registered by
WMShll#initOneHanded, then the NPE will happen.

The simple fix is to add NPE check in
OneHandedController#notifyExpandNotifcation()
we can just ignore the callback during init time since the singal
is to expand notification and come from shorcut after user enable
and tap shortcut.(No need to act for the signal during boot progress.)

Test: manual reboot device and observe
Test: atest WMShellUnitTests
Bug: 191600033
Change-Id: I73849afa9904031759da304298221dbb222aeaaa
(cherry picked from commit c4bdf37a)