AglShellGrpcClient: Add wait_for_ready + deadline for grpc::Context

AglShellGrpcClient: Add wait_for_ready + deadline for grpc::Context

It seems that gRPC channel might not be ready when issueing floating
request so add a wait_for_ready + deadline to make sure we don't send
the request until the channel is available. Initial testing (though we
might need more) shows that this fixes the issue.

The way this works is that the client (if started with wait_for_ready
set to true, passed in the AglShellGrpcClient constructor) will wait for
the channel to be ready and then issue a floating request. There's a
hidden deadline of 2 seconds in case that doesn't happen.

Bug-AGL: SPEC-5235
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I40a5ad0dd760dc4559839ae7c302c1d6bc1ff973