Add utils to convert Inet4Address <-> int

The current utilities map the higher-order bytes of ints to the
lower-order bytes of IPv4 addresses, which makes applying masks and
generating series of addresses confusing.

For example, the current utilities would convert 1.2.3.4 to 0x04030201,
or generate 0x0080ffff for a /17 subnet mask. The utilities in this CL
convert 1.2.3.4 to 0x01020304, and a /17 subnet mask is represented as
0xffff8000.

Includes tests for all the above methods and migration of
NetworkUtilsTest to JUnit4.

Test: added tests pass
Change-Id: I5f5aa2e6e3b054b66b9dc507dab64f41be9139b1