Add server side proto-base dumping for back navigation

This CL implements a mechanism to dump the state of back navigation
in BackNavigationController to a proto buffer which we can easily
check the state while runing test case.

This also add @TestApi for BackNavigationInfo.

Bug: 131727607
Test: atest BackNavigationGestureTest
Change-Id: I8918f0b8095177578e23eb92547bd705ccfa7ec1