Add heart beat event to Default VHAL.

VHAL needs to send out heartbeat event so that watchdog does not
kill it. This is required for all VHAL so move the logic to default
VHAL.

Test: Local run on emulator. Add a test case in
android.hardware.automotive.vehicle@2.0-default-impl-unit-tests

Bug: 193831021
Change-Id: Ieacd991c22fa0d70d254426d728d3827a38d0b42