Versioning for apex init.rc files

Support an "init.#rc" versioning scheme for apex init files.
chooses highest # <= current system sdk.
".rc" (aka the old init.rc) is treated as sdk 0
Document these semantics in README.md

Bug: 198186200
Test: booting, lots of logcat output
Change-Id: I2d0405be73dae9bafa6f22535b29ed6b053ddbc4