Add support for mocked RPMB

When developing, it may be preferable to operate on a device which does
not have a real RPMB storage, or which is unprovisioned. This CL allows
the rpmb_dev program to act as a daemon serving a fixed key, and for
storageproxyd to speak to rpmb_dev's socket rather than an actual rpmb
device or a virtual rpmb device.

Test: Trusty Gatekeeper VTS
Change-Id: I19b2b143fffb8e68e4a028d00eaf5cd1928e12f6