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This CL adds a HostSettings class and provides an implementation for Mac
for reading simple key-value pairs, and add a new host setting,
`capture_audio_for_device_uid` to specify which UID the Mac host should
capture the audio from.

This code is still very preliminary. It only loads the file once, and it
doesn't create the file for the user. The user will need to manually
create a file at:
/Library/PrivilegedHelperTools/org.chromium.chromoting.settings.json,
make it readable by both root and the current $USER, and add the
capture_audio_for_device_uid mapping.

Bug: 1161363
Change-Id: I8ee5269908a9a775d003b6349e3c7567d88380b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612400
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841285}