mm-video-v4l2: venc: support setting bitrate savings

Add support to enable/disable bitrate savings through
platform config.

CRs-Fixed: 2347481
Change-Id: Idf628f2ba6b827731918104d0f96c6830b7e3a7a
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>

• conf_files/msmnile/system_properties.xml[diff]
• libplatformconfig/PlatformConfig.h[diff]
• mm-video-v4l2/vidc/venc/inc/video_encoder_device_v4l2.h[diff]
• mm-video-v4l2/vidc/venc/src/video_encoder_device_v4l2.cpp[diff]