Handle null from unitialized live data

In the onPermissionUsagesChanged() callback we weren't checking whether
getPackagePermGroupsLiveData().getValue() was null. However, we were
checking in another instance. Make usage consistent by annotating the
parameter as Nullable and properly handling null.

Bug: 205195921
Bug: 216693713
Test: Manual testing
Change-Id: I6d4a2b79492ab4fa5a3df0f3f6b1d85a1331d8e5
(cherry picked from commit 6ce5f9ec)