init-debug.rc: add runtime mmc max speed property actors

persist.mmc.max_read_speed  - KB/s speed cap
persist.mmc.max_write_speed - KB/s speed cap
persist.mmc.cache_size      - MB of high-speed on-device write cache

device default directly to associated /sys/block/mmcblk0/ attributes,
not delivered on "user" builds.

Bug: 26976972
Change-Id: Ic82b9614489b6406ce830d127c85affc9095e256