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

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