fskeyring & userspace reboot: support DE keys

During userspace reboot /data might be unmounted, which means that if
device supports filesystem keyring, DE keys will be lost and are needed
to be re-installed.

Test: adb shell setprop sys.init.userdata_remount.force_umount 1
Test: adb shell svc power reboot userspace
Test: atest CtsUserspaceRebootHostSideTestCases
Bug: 143970043
Change-Id: I153caa1d7c373b3c906a34f1184c681e52854a9d
Merged-In: I153caa1d7c373b3c906a34f1184c681e52854a9d
(cherry picked from commit 1eaea5a6)