Support atoms subscription via shell cmd.

+ This feature is for supporting perfd.
+ Perfd is built outside android and doesnot depend on any android libraries.
  So the communication between perfd and statsd can only be via non-android IPCs
  (such as socket, pipe, file descriptors etc.)
+ Perfd runs as a shell user, so using the existing shell cmd is a natural choice.
+ The input is a simple config, and output is a stream of atoms proto binary data.

+ Also cleaned up the code so that we use file descriptor directly instead of creating another
  FILE*.

TODO: pulled atom subscription.

Bug: 110536553
Test: statsd_test and manually tested
Change-Id: I64b0061cc66b5f7648147885a2ac1af531c19917