Refactor audio_processing/agc: Removes usage of macro WEBRTC_SPL_MUL_16_16_RSFT
Refactor audio_processing/agc: Removes usage of macro WEBRTC_SPL_MUL_16_16_RSFT

The macro is defined as
#define WEBRTC_SPL_MUL_16_16_RSFT(a, b, c) \
(WEBRTC_SPL_MUL_16_16(a, b) >> (c))

where the latter macro is in C defined as
#define WEBRTC_SPL_MUL_16_16(a, b) \
((int32_t) (((int16_t)(a)) * ((int16_t)(b))))
(For definitions on ARMv7 and MIPS, see common_audio/signal_processing/include/spl_inl_{armv7,mips}.h)

The replacement consists of
- avoiding casts to int16_t if inputs already are int16_t
- adding explicit cast to <type> if result is assigned to <type> (other than int or int32_t)
- minor cleanups like remove of unnecessary parentheses and style changes

BUG=3348,3353
TESTED=locally on Mac and trybots
R=kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47449004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8664}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1afbdc7555b4bcf9da3bc12ed60e32a0ac0dd297

sippet/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,Alkalyne/webrtctrunk,sippet/webrtc,sippet/webrtc,aleonliao/webrtc-trunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,krieger-od/webrtc,Alkalyne/webrtctrunk,aleonliao/webrtc-trunk,krieger-od/webrtc,aleonliao/webrtc-trunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,sippet/webrtc,krieger-od/webrtc,sippet/webrtc,jchavanton/webrtc,krieger-od/webrtc,aleonliao/webrtc-trunk,krieger-od/webrtc,krieger-od/webrtc,jchavanton/webrtc,aleonliao/webrtc-trunk,aleonliao/webrtc-trunk