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