[vfs] Prepare for adding to the SDK

This CL fixes several compiler errors that are specific to the SDK.

This CL also adds a readme that is explicit about the current scope of
this library and which parts are likely to change in the future.

Test: These changes are required to pass the SDK bots.
Change-Id: I3e3a99e2ae8e9d6fadd087959a3c4e9a9d6120d0