init: non-crashing service can restart immediately

This CL allows restart_period to be set to a value shorter than 5s.
Previously this was prohibited to rate limit crashing services. That
behavior is considered to be a bit too conservative because some
services don't crash, but exit deliverately.

adbd is the motivating example. When adb root or adb unroot is
requested, it changes its mode of operation (via sysprop), exits itself,
and restarts (by init) to enter into the mode. However, due to the 5s
delay, the mode change can complete no earlier than 5 seconds after adbd
was started last time. This can slow the mode change when it is
requested right after the boot.

With this CL, restart_period can be set to a value smaller than 5. And
services like adbd can make use of it. However, in ordef to rate limit
crashing service, the default is enforced if the service was crashed
last time. In addition, such intended restart is not counted as crashes
when monitoring successive crashes during booting.

Bug: 286061817
Test: /packages/modules/Virtualization/vm/vm_shell.sh start-microdroid \
 --auto-connect -- --protected
* with this change: within 2s
* without this change: over 6s

Change-Id: I1b3f0c92d349e8c8760821cf50fb69997b67b242