Fix AddNetworkFragment bottom buttons position problem

AddNetworkFragment bottom buttons align parent bottom,
it has display problem because collapsing toolbar changes
its height and squeeze the bottom buttons out of the screen.

This change sets the bottom buttons align the bottom of
other content.

Bug: 179656154
Test: manual visual
Change-Id: I6f0062d10c603e40e3f8c2683d5610418df6522a