Rename EntropyService to EntropyMixer

EntropyService implies that this program provides entropy to other
programs, and is misleading. The EntropyMixer class is designed purely
to stir the existing entropy pool with some possibily random-ish data,
and carryover entropy across device reboots.

Change-Id: I086cd339a3b652d32371521e61e1b1f555ce2280