src: server: Allocate buffers with GRALLOC_USAGE_HW_VIDEO_ENCODER

Certain OMX video encoder implementations might require the buffer pixels to be arranged fit for the encoder when using Aethercast.

To give the encoder what it needs, just allocate server-side (and only server-side) buffers using this flag for correct presentation.

Also see: mir-android-platform!36 (closed)

Keeping as a draft for now while I test on Halium 9 Volla Phone (yggdrasil) with UT 20.04 😄