Add support for updatable services

A service with 'updatable' option can be overriden by the same service
definition in APEXes.

/system/etc/init/foo.rc:

service foo /system/bin/foo
    updatable

/apex/myapex/etc/init.rc:

service foo /apex/myapex/bin/foo
    override

Overriding a non-updatable (i.e. without updatable option) service
from APEXes is prohibited.

When an updatable service is started before APEXes are all activated,
the execution is delayed until when the APEXes are all activated.

Bug: 117403679
Test: m apex.test; adb push <built_apex> /data/apex; adb reboot
adb shell, then lsof -p $(pidof surfaceflinger) shows that
the process is executing
/apex/com.android.example.apex@1/bin/surfaceflinger instead of
/system/bin/surfaceflinger

Change-Id: I8a57b8e7f6da81b4d2843e261a9a935dd279067c