graceful shutdown in microdroid

This change allows the shutdown watchdog timeout to be arbitrarily small
(ex: 2s) in Microdroid.

Previously, this was set to 60 sec for both Android and Microdroid.
Microdroid however actually wasn't affected by this because it never
implemented graceful shutdown.

Now, Microdroid tries to gracefully shutdown whenever it can. If a VM is
no longer needed, instead of powering the cable off (by killing the
crosvm process), we now send a command to Microdroid to initiate a
shutdown sequence by setting sys.shhutdown.requested.

On the Android side, if the complete shutdown is not observed withint 5
seconds, it forcibly kills the VM, in which case data corruption may
happen.

Without this change, the voluntary shutdown happens only after 60
seconds, however, this is unnecessarily long for Microdroid. Unlike
Android, it doesn't have big/many disks to sync. Only encrypted storage
needs to be synced and this can be done faily quickly.

To alleviate this, Microdroid additionally set the shutdown watchdog
timeout to 2 seconds. And this change allows such a short timeout for
Microdroid.

Bug: 419607474
Flag: EXEMPT bug fix
Test: see the other commit in the same topic

Change-Id: I15553c3ca9cef80d7b3adc5690a151442a335408