Create new sysprops module

New module loads sysprops defaults from config file and merges
config fragments from the corresponding .d directory if it exists.

This allows non-Android OS's to use sysprops and gives them a way
to set and override sysprop defaults.

Also migrates bluetooth.core.gap.le.privacy.enabled on Linux to
use the new module.

Bug: 233119381
Bug: 233119695
Tag: #floss
Test: Unit tests and manually verified privacy sysprop
Change-Id: I7167f6fdfa9ec2ec3c1504b8b44f92c5ae2ba174