userspace reboot: don't re-install keyring when remounting userdata

From the implementation of FscryptInstallKeyring it looks like it will
install a new keyring on each call.

Another approach would be change FscryptInstallKeyring to first call
request_key[0] (http://man7.org/linux/man-pages/man2/request_key.2.html),
and don't add keyring in case request_key succeeded, but it looks like
that libkeyutils doesn't provide such functionality.

Since I was there, I've also added checks that we are not trying to
remount userdata on FDE devices.

Test: adb reboot userspace
Bug: 135984674
Change-Id: I2e063d7d87a3c2c26810e913a33e3a5c0364332b