Use the HWC caching mechanism to avoid stalls in the ion driver.

HWC supports caching buffers for layers using "slot" assignments.
Use this in VrFlinger to avoid importing a buffer handle every
frame. The avoids periodic stalls we observe in the ion driver
when mapping a buffer into the HWC address space.

Bug: 66459419
Test: Observe systraces no longer have MapBuffer in HWC in steady
      state; system does not drop frames.

Change-Id: Iba4161b33561322bfbccbfafe600b432a6fa7c44