Add support for preconnected RPC client

Normally we want to call RpcClient directly, but the apps don't have
access to create a client vsock themselves. This change adds a helper
function to establish an RPC session with preconnected file descriptor.

Bug: 195381416
Test: atest MicrodroidHostTestCases
Change-Id: Idbdc6b00ca459ed10b1c79e64f004708594b92da