gps: report session begin/end on gps start/stop

Since modem does not consistently send
SESSON_END/ENGINE_OFF event, always report
SESSION_END on stop gps and SESSION_BEGIN on
start gps so that the icon is turned on/off
correctly.

CRs-fixed: 401664
Change-Id: Ib3954529056a653d7057aedb489be1ccee9eaf1d
(cherry picked from commit dc7abf9a3d621b7a744e0eaeb6d9819ffff869a5)