Make BundleKey a distinct struct from PacketTrace.

This change refactors BundleKey to be a separate struct. This allows
us to do the conversion (e.g. of sport/dport, or icmp fields in the
future) in the constructor and change types (use optionals) where it
is convenient to simplify the code.

This change also fixes a bunch of desigated initializer order warnings
in the test (fields should be declared in same order as defined).

Test: atest libnetworkstats_test
Change-Id: I6373d2512280e93c54395fde38dcff43705b449e