On Ubuntu 18.04, /bin/sh is symlinked to dash. Executing codegen under
dash results in the following error:
codegen: Syntax error: "(" unexpected
To prevent this, explicitly use bash.
Test: manual ($ANDROID_BUILD_TOP/development/scripts/codegen --help)
Change-Id: I11bd5e41757a8d26f5e9d7d3340306d5ff4da1f9
Exempt-From-Owner-Approval: introduces separate utility; doesn't touch anything around it
Test: 1. Manually enter version/presence of build artifact,
and ensure `codegen FILENAME` still works
2. run codegen again without altering version, ensure no build triggered
Change-Id: Iaa253a430e627d0e7c25781cde535958f77db97f
