Don't try caching null ringtones.
Don't try caching null ringtones.

Bug: 27330629
Change-Id: Id09d10d6f4a3a20782480647c68a5c94ef69e7b3

xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base,xorware/android_frameworks_base