Move TapPacketReader into its own utility class.

This class is generally useful for tests that manipulate packets
using a TestNetworkInterface. It will be used in upcoming
tethering tests. Also make it take a FileDescriptor instead of a
ParcelFileDescriptor, since that is more generally more useful.

As part of this change, create a filegroup of files that the test
utilities can depend on.

Bug: 150644681
Test: atest NetworkStackIntegrationTests:IpClientIntegrationTest
Change-Id: I73e9c1919956aa665a8cccec11d3444e486426ec