Relocate KeepalivePacketData to frameworks/base

Due to an issue resolving the boot classpath, the
KeepalivePacketData structure cannot be referenced
by frameworks/opt/telephony while it is in services.

-Move KeepalivePacketData to android.net
-Also, relocate IpUtils without changing the package
 name.

Bug: 38350389
Test: compilation
Change-Id: If5fc63e9ad8b9b2d4c2fee47ff4bab2ab190a05a
(cherry picked from commit bd4093be7cab51d6ad031854324134a9c8637c2c)