Adding Multiple Client Support for ShellSubscriber

Allowing multiple ShellSubscriber clients. Splitting the ShellSubscriber
class into ShellSubscriber and ShellSubscriberClient classes.
ShellSubscriberClient handles the pushing and pulling of atoms and
ShellSubscriber spawns ShellSubscriberClients for consumers.

Bug: 242313538
Bug: 236673059
Test: statsd_test64
Test: ShellSubscriberTest
Change-Id: I53be07fdc85f71713ece23e3c78e2a36c7c5fbd6