Add another purchase button that uses a key which isn't invalidated even if a
new fingerprint is enrolled.

This CL includes the changes in the previous http://ag/913148 and the change
to fix the build by "make FingerprintDialog" (The previous http://ag/913148
broke the build so was reverted)

Change-Id: Ia2dd793ce041b4720b7c7105041e942d82a35f86
6 files changed