aacenc: add AAC_CODER_(FAAC|ANMR|etc.) macros

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
aacenc: use constants to set AAC coder

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/aacenc: use enum for aac coder.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>