Support adding NATT keepalive packet filter

Support adding NATT keepalive packet filter to APF
filter.
Generating APF program will be addressed in another CL.

Bug: 33530442
Test: - atest NetworkStackTests
      - atest FrameworksNetTests

Change-Id: I403cd14ac9aa6b001c4e580abbb33a615931a192
Merged-In: Idaa7238a5c9acdae9f6cff13095ee9436c7c92c8
(cherry picked from commit 038c11d564452c9e08f25119423049339ff93c57)

• packages/NetworkStack/src/android/net/apf/ApfFilter.java[diff]
• packages/NetworkStack/src/android/net/ip/IpClient.java[diff]
• packages/NetworkStack/tests/src/android/net/apf/ApfTest.java[diff]
• services/net/java/android/net/ip/IIpClient.aidl[diff]
• tests/net/java/com/android/server/ConnectivityServiceTest.java[diff]