Remove ParcelableString references

Even better would be to completely get rid of non-framework code using
this, and then even better than that would be to completely get rid of
all code using this.

Test: manual. open Settings > Security > Trusted Credentials before and after this change. Number of certificates should be the same. No crashes should happen in either case.
Change-Id: I4237e630b903eb04e6b572d6464a40d183b24276