Clarify how to replace cert generated by Android Keystore.

This updates Android Keystore developer documentation to clarify how
to replace the self-signed certificate create by Android Keystore when
it generates a new key pair. Some developers are attempting to use
KeyStore.setCertificateEntry which is the wrong method for this. The
correct method is KeyStore.setKeyEntry.

Bug: 28152878
Change-Id: I306447b7792ecad5fbb49bd691a57bedb5207003