Add vr persistent mode enable/disable

Persistent vr mode toggle control is needed sometimes from command line.
This also adds the infrastructure for future vr commands.

Bug: 36071574
Test: adb shell vr set-persistent-vr-mode-enabled true
      adb shell dumpsys vrmanager
      >> Persistent VR mode is currently: enabled
      adb shell vr set-persistent-vr-mode-enabled false
      adb shell dumpsys vrmanager
      >> Persistent VR mode is currently: disabled

Change-Id: I47a858e0696a907e746d50e2c6ca3d18197864a7
Signed-off-by: Karthik Ravi Shankar <karthikrs@google.com>