Make VR mode virtual display properties customizable

Currently the virtual display created in VR mode is of fixed size. This
may not work for all the devices. Add APIs to make the resolution and
DPI adjustable from VrCore.

Bug: 37260266
Test: Sample reflection which works - cl/153039188
Test: adb shell set-persistent-vr-mode-enabled true
      adb shell vr set-compatibility-display-properties 500 500 500
      adb shell dumpsys display
      >>   DisplayDeviceInfo{"VR 2D Display":
      uniqueId="virtual:android,1000,VR 2D Display,0", 500 x 500, modeId
      6, defaultModeId 6, supportedModes [{id=6, width=500, height=500,
      fps=60.0}], colorMode 0, supportedColorModes [0], HdrCapabilities
      null, density 500, 500.0 x 500.0 dpi, appVsyncOff 0, presDeadline
      16666666, touch NONE, rotation 0, type VIRTUAL, state ON, owner
      android (uid 1000), FLAG_PRIVATE, FLAG_NEVER_BLANK,
      FLAG_OWN_CONTENT_ONLY}

Change-Id: Ibb2d03b7f7f8f62fcce97fcc2b373ae3f09cef01
Signed-off-by: Karthik Ravi Shankar <karthikrs@google.com>