Handle null case KeyChain getPredefinedAliasForPackageAndUri

Bug: 177999278
Test: Manual testing

Manual Testing Steps:
* Install TestDPC
* Select 'Request to manage credentials'
* Select 'OK' then 'Allow'
* Select 'Install a private/public key pair'
* Select a certificate and enter name 'testAlias'
* Select 'Remove a private/public key pair'
* Verify NullPointerException is not thrown

Change-Id: If8019baeab90ca75e701482b3090c041147f5a89