Prevent using abstract terms in the property name

In many cases, it's a common practice to use the name of the
domain type of a process or the type of the usage category
instead of using ambiguous terms like sys, system, etc.
Update the property name with net. prefix for better naming to
fit the usage of the system property.

Bug: 170917042
Test: 1. m -j10
      2. Check if /proc/sys/net/ipv4/tcp_default_init_rwnd is
         updated as expected
Change-Id: I0267880d62cc504a419827732780d2db97b2dfef