Add NPE check for method assignDefaultPhoto in Utils.java

Add NPE check for method assignDefaultPhoto in Utils.java

When user press back button while adding user, app may crash because
of null context. In this cl, I add NPE check for context in method
assignDefaultPhoto and change this method to return boolean value.
If the return value is false, then stop the following steps(e.g.
creating a dialog after pressing button may also cause crash.)

Bug: 33829308
Test: make -j40 RunSettingsRoboTests
Change-Id: I811b92c268fb20f8c43ab3b0a9ceebe76f5d83be