JSIndexedRAMBundle: Don't read null bytes for scripts

Summary:
`JSIndexedRAMBundle` used to read scripts including the terminating null bytes. That worked on iOS, but not on Android, where we have an ascii-only optimization.
This changes that behavior to only read the actual script data excluding the terminal null.

Reviewed By: javache

Differential Revision: D4265374

fbshipit-source-id: 7e6f943666aee610d79939cef09b103305803c69