minadbd sends heartbeat to rescue service for getprop command.

We start minadbd and rescue services in two processes. In particular,
minadbd handles the requests from host, then communicates with rescue
service to do install/wipe works. When resuce service doesn't see any
request in a pre-defined timeout (currently 300s), rescue service will
exit to avoid endless waiting.

This CL changes minadbd to additionally send a no-op command to rescue
service as a heartbeat signal, so that host side can finish
time-consuming operations (e.g. downloading over network) while keeping
rescue service alive.

Bug: 136457446
Test: Enter resuce mode on blueline. Send `adb rescue getprop
      ro.build.fingerprint` and check that rescue service doesn't exit.
Test: Stop sending the getprop command. Check that rescue service exits
      after 300s.
Change-Id: Ib9d5ed710cfa94ecfe6cf393a71a0b67b2539531