stagefright amrnb: Remove extern from table definitions

Local prototype declarations are added where no suitable ones are
available in headers.

This fixes warnings when compiling as C - while the source
files are named .cpp, they actually are compileable as pure C.

This was committed on the master branch of the opencore
repository in commit a0acb494ea6682ca966b5ebbd3676c89c56d63a6,
based on AOSP contribution 10303.

Change-Id: Ib4e27ea30302c66e0233e63e5a4652f6339bc940