SystemUI: Set zen mode to no interruptions if silent mode is set

When setting ringer mode to silent and current zen mode is off, zen
mode will become priority interruptions mode. As a result, ringer mode
is set to normal mode in updateZenMode.

Set zen mode to no interruptions when ringer mode is set to silent.

CRs-Fixed: 744813

Change-Id: Id104e5473d0e8d624b06dfc9e5dbd8d9f7627b6e