Refactor common_audio/vad: Removed usage of trivial macro WEBRTC_SPL_MUL_16_16(a, b)
Refactor common_audio/vad: Removed usage of trivial macro WEBRTC_SPL_MUL_16_16(a, b)

The 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)

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

Review URL: https://webrtc-codereview.appspot.com/43129004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#8986}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 61a4b04f4004edb91e9ee6b76d4d01971a795ed6

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