Replace generated files with a genrule

Required for an update to external/flatbuffers (see other CLs in the
topic). This ensures generated header stays up-to-date with the
external/flatbuffers.

Checked in sources caused a build failure after updating flatbuffers
library, as they contain hard-coded check against the library version.

Bug: 391097044
Test: lunch aosp_cheetah-trunk_staging-userdebug && mm
Change-Id: I9da76a26581c30c5c7dad46cc7b0ff2575b5005b