CtsAlarmManagerTestCases: Care for batteryless devices.

From P, the default battery status is "unknown" for batteryless
devices. Just sending 'dumpsys battery unplug' without setting a
valid battery status does not fake the system to be on battery.

Adding the additional shell command will ensure the whole test
is run under mocked discharging state, regardless of the real
device status.

Bug: 126761521
Test: CtsAlarmManagerTestCases on Chromebase (desktop) and phones
Change-Id: I9f301e31897d230a8fa6f8f854d95f58834784f1
diff --git a/tests/AlarmManager/src/android/alarmmanager/cts/AppStandbyTests.java b/tests/AlarmManager/src/android/alarmmanager/cts/AppStandbyTests.java
index 8d8adb1..8838492 100644
--- a/tests/AlarmManager/src/android/alarmmanager/cts/AppStandbyTests.java
+++ b/tests/AlarmManager/src/android/alarmmanager/cts/AppStandbyTests.java
@@ -299,9 +299,12 @@
 
     private void setBatteryCharging(final boolean charging) throws Exception {
         final BatteryManager bm = mContext.getSystemService(BatteryManager.class);
-        final String cmd = "dumpsys battery " + (charging ? "reset" : "unplug");
-        executeAndLog(cmd);
-        if (!charging) {
+        if (charging) {
+            executeAndLog("dumpsys battery reset");
+        } else {
+            executeAndLog("dumpsys battery unplug");
+            executeAndLog("dumpsys battery set status " +
+                    BatteryManager.BATTERY_STATUS_DISCHARGING);
             assertTrue("Battery could not be unplugged", waitUntil(() -> !bm.isCharging(), 5_000));
         }
     }