Don't crash in Music Storage Settings.

This crash occurs because the music files entry is active, but does
not have an AppEntry (because it is not an app.) ManageApplications
assumes all active elements have an AppEntry, which causes an NPE.

By adding in a null check, we can avoid this.

Bug: 34888282
Test: Manual -- because ManageApplications needs refactoring badly.
Change-Id: Ief0c202f6176d90e492e62e5930b940e5a28b95c