Let init mount/unmount debugfs for non-user builds

This change will help non-user builds with keeping debugfs
disabled during run time. Instead, debugfs will be mounted by init
to enable boot time initializations to set up vendor debug data
collection and unmounted after boot. It will be also be mounted by
dumpstate for bug report generation and unmounted after.

This change is only intended to help vendors (who depend on debugfs to
collect debug information from userdebug/eng builds) keep debugfs
disabled during runtime. Platform code must not depend on debugfs at all.

Test: manual
Bug: 176936478
Change-Id: I2e89d5b9540e3de094976563682d4b8c5c125876