Make specifying self-signed cert parameters optional.

This removes the need to specify the three parameters of the
self-signed certificate (serial number, subject, validity range) when
generating key pairs in AndroidKeyStore. This is achieved by
providing sensible defaults for these parameters:
* serial number: 1
* subject: CN=fake
* validity range: Jan 1 1970 to Jan 1 2048.

Bug: 18088752
Change-Id: I5df918b1ef8b26ed3ddd43828c4c78c9fa58cd43