Keep standalone devices in VrFlinger mode.

Avoid unnecessary tear-down and bring up of HWC client in standalone
devices. This saves time during screen-on because standalone devices
only use VrFlinger mode.

Remove dead code that used to check the panel driver for missed frames.
No drivers expose the sysfs node that provided this function anymore.

Test: Manual testing. Observe stable DON/DOFF behavior.
Bug: 65248224
Change-Id: Id9ebb76982621848d97792496a09b6da8c4e5928