[NAN] Refactor WifiNanManager to create separate sessions per connect. [DO NOT MERGE]

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

Bug: 31470256
Test: unit tests, integration (sl4a) tests.
Change-Id: I83d69acf18de6be2b96a34d2a608a6390ecdecbf
9 files changed