Check if DUN APN exists to decide isDunRequired

Change isDunRequired variable from integer to boolean.
Before this change tethering would reference
config_tether_upstream_types reource configuraton to decide
isDunRequired for DUN_UNSPECIFIED case.
After the change, if isDunRequired is true, tethering would
request DUN. If it is false, tethering skip DUN type. In order
to backwards compatibility, tethering would not add mobile/hipri
type to upstream configuration if one of them already exist in
upstream configuration.

Test: - build, flash, booted
      - runtest frameworks-net
      - manully test with TMO, VZW SIM.
bug: 63150712

Change-Id: Ia998ea502674ff186d52af37e3fd8f07de495245