Gallery2: Fix NPE in LocalSource during monkey test.

The object which is obtained by accquire MediaProvider client is null
when MediaStorage is killed or disabled by monkey test or others.

Add null check in LocalSource.java::pause().

CRs-Fixed: 540075

Change-Id: Icba85e77e86793e344e55255c27b195c11186fc8
Merge "Gallery2: Fix NPE in LocalSource during monkey test." into du44