[NAN] Refactor WifiNanManager to create separate sessions per connect.

Separate session enables separate namespace for functions which depend
on a connection to exist.

(cherry-pick of commit a2eb21d1233e921487e3dada1d10ba3e14611168)

Bug: 31470256
Test: unit tests, integration (sl4a) tests.
Change-Id: I8be0bc609e55319446cef149ba49cd6f7a5f446a
9 files changed