Minor fixes on VTS tests

- call startSvStatus()/startNmea() before calling start(). This will
  avoid missing the first SvStatus
- in TestGnssSvInfoFieldsTest, avoid calling list::back() if the list is
  empty.

Bug: 235062809
Test: atest VtsHalGnssTargetTest
Change-Id: I612d42e23085dbb0b32da328869bfd443ee66362
diff --git a/gnss/aidl/vts/gnss_hal_test.cpp b/gnss/aidl/vts/gnss_hal_test.cpp
index 407ac0c..0e1218e 100644
--- a/gnss/aidl/vts/gnss_hal_test.cpp
+++ b/gnss/aidl/vts/gnss_hal_test.cpp
@@ -141,18 +141,18 @@
     }
     SetPositionMode(min_interval_msec, low_power_mode);
 
-    auto status = aidl_gnss_hal_->start();
-    EXPECT_TRUE(status.isOk());
-
     if (start_sv_status) {
-        status = aidl_gnss_hal_->startSvStatus();
+        auto status = aidl_gnss_hal_->startSvStatus();
         EXPECT_TRUE(status.isOk());
     }
     if (start_nmea) {
-        status = aidl_gnss_hal_->startNmea();
+        auto status = aidl_gnss_hal_->startNmea();
         EXPECT_TRUE(status.isOk());
     }
 
+    auto status = aidl_gnss_hal_->start();
+    EXPECT_TRUE(status.isOk());
+
     /*
      * GnssLocationProvider support of AGPS SUPL & XtraDownloader is not available in VTS,
      * so allow time to demodulate ephemeris over the air.