Fix usage of HWC_DISPLAY_PRIMARY in vr flinger

The hardware composer functions require display ids obtained from the
onHotplug() composer callback. Our vr flinger code was supplying
HWC_DISPLAY_PRIMARY as the primary display id, which is incorrect. The
HWC_DISPLAY_* values are used for representing the display type, not the
display id. The code worked anyway because most hardware composers
always use 0 as the primary display id, which happens to be the same
value as HWC_DISPLAY_PRIMARY. The emulator uses 1 as the primary display
id though, which exposed the bug.

This CL changes the vr flinger code to get the display id from the
onHotplug() composer callback, and uses that value when talking to
hardware composer, instead of HWC_DISPLAY_PRIMARY. This matches the
behavior of surface flinger.

Bug: 69631196

Test: Verified the vr flinger code path works as expected on phones and
standalones.

Change-Id: Ia691941d0eafaa1f89e0ee81a4ae27fdef5a57cf