On standalones, stay in vr mode until standby

On standalones, stay in vr mode until the Power Policy Service notifies
us it has switched to standby mode.

We still have a few issues elsewhere in the code, so the "stay in vr
mode" functionality is only enabled if the system property
"persist.vr.use_standby_to_exit_vr_mode" is set to true, which you can
do like this:

$ adb shell setprop persist.vr.use_standby_to_exit_vr_mode true

Bug: 65248224

Test: - Confirmed on a standalone that doffing leaves us in vr mode for
5 minutes, then we get a notification that we've entered standby, and we
exit vr mode. Confirmed we enter vr mode again when we don the device.

- Confirmed no behavioral change on phones, as expected.

Change-Id: I41d62761fe8c2fc7630f0615732453669cb25f06