[Sb refactor] Ensure config is fetched on flow start

In the case where the `CARRIER_CONFIG_CHANGED` broadcast is sent out
before the default data sub RAT config is read, it will read the
system-defined default carrier config instead of the appropriate one for
the current default data sub.

This change is simple: make sure to always fetch the system current
carrier config when the flow starts.

Bug: 280390475
Test: MobileConnectionsRepositoryTest
Change-Id: I868a840ee8465b0313bc85e5c35c0d40ca9d509a

packages/SystemUI/src/com/android/systemui/statusbar/pipeline/mobile/data/repository/prod/MobileConnectionsRepositoryImpl.kt

@@ -233,6 +233,7 @@ constructor(
 
     override val defaultDataSubRatConfig: StateFlow<Config> =
         merge(defaultDataSubIdChangeEvent, carrierConfigChangedEvent)
+            .onStart { emit(Unit) }
             .mapLatest { Config.readConfig(context) }
             .distinctUntilChanged()
             .onEach { logger.logDefaultDataSubRatConfig(it) }

packages/SystemUI/tests/src/com/android/systemui/statusbar/pipeline/mobile/data/repository/prod/MobileConnectionsRepositoryTest.kt

@@ -1111,6 +1111,33 @@ class MobileConnectionsRepositoryTest : SysuiTestCase() {
             assertThat(latest!!.showAtLeast3G).isTrue()
         }
 
+    @Test
+    fun carrierConfig_initialValueIsFetched() =
+        testScope.runTest {
+
+            // Value starts out false
+            assertThat(underTest.defaultDataSubRatConfig.value.showAtLeast3G).isFalse()
+
+            overrideResource(R.bool.config_showMin3G, true)
+            val configFromContext = MobileMappings.Config.readConfig(context)
+            assertThat(configFromContext.showAtLeast3G).isTrue()
+
+            // WHEN the change event is fired
+            fakeBroadcastDispatcher.registeredReceivers.forEach { receiver ->
+                receiver.onReceive(
+                    context,
+                    Intent(CarrierConfigManager.ACTION_CARRIER_CONFIG_CHANGED)
+                )
+            }
+
+            // WHEN collection starts AFTER the broadcast is sent out
+            val latest by collectLastValue(underTest.defaultDataSubRatConfig)
+
+            // THEN the config has the updated value
+            assertThat(latest!!.areEqual(configFromContext)).isTrue()
+            assertThat(latest!!.showAtLeast3G).isTrue()
+        }
+
     @Test
     fun activeDataChange_inSameGroup_emitsUnit() =
         testScope.runTest {