Use proper string formatting for the "Active device" string

Updated testContextMock() how it checks the bluetooth_connected string.

Bug: 72317067
Test: Unit test updated:
      make ROBOTEST_FILTER=BluetoothDetailsHeaderControllerTest \
                           RunSettingsRoboTests
      Manual: two headsets and switching the active device
Change-Id: I3db178d71543e4dfa437544350c58241860ae703
diff --git a/tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsHeaderControllerTest.java b/tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsHeaderControllerTest.java
index ab338a1..0129b1c 100644
--- a/tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsHeaderControllerTest.java
+++ b/tests/robotests/src/com/android/settings/bluetooth/BluetoothDetailsHeaderControllerTest.java
@@ -80,7 +80,7 @@
      */
     @Test
     public void testContextMock() {
-        assertThat(mContext.getString(R.string.bluetooth_connected)).isEqualTo("Connected");
+        assertThat(mContext.getString(R.string.bluetooth_connected)).isNotNull();
     }
 
     @Test