Avoid SysUI force closing when enable/disable DSB System bar gradient option

Change-Id: I5d6dcea9f4d7187a9c14d8ef0dd5c1c874cb8d9f
SystemUI: Fix crash during notification/theme change

Fixes crashing that could occur when a notification is created
during a theme change. Entries were attempting to be added
to views which did not have a layout pass yet, causing it to fail.

When a theme change occurs, the fix is to halt the ticker and command queue
until a layout occurs.

BUGDUMP-151925

Change-Id: Ic19d26178df673a1d52981a8617415e6fbcb219d