HFP: Use SignalStrength#getLevel() to get signal level

* According to TS 127 007 v6.8.0 Section 8.9, +CIND indicator value for
  "signal" should be 0 when "the indicator is off (or in state which can
  be identified as 'off'-state)". Therefore, this CL only assigns +CIND
  "signal" indicator as 0 when cellular network is unavailable
* When network state is available, call SignalStrength#getLevel() to get
  network signal level in the range of 0 to 4 and add 1 to this level to
  get +CIND signal level

Bug: 109857475
Test: make, connect to car kit at different network scenario
Change-Id: I9c737813fd779351efc387ca14eb41b56d6e02e2
(cherry picked from commit 340b4aa1)