3/ Fix bug of PipKeyboardTest do not forceExit() testApp successfully

After PipKeyboardTest teardown{}, the testApp in PIP mode is alive and
introduce the NEXT flicker test case get unexpected assertion fails.

Root cause:
The flow of exit(), neither pressHome() nor pressBack() could lead
TestApp(in PIP) successfully exit PIP and back to fullscreen in teardown.

Bug: 173477410
Test: atest com.android.wm.shell.flicker, make sure all tests PASSED
Test: atest WMShellFlickerTests, make sure all tests PASSED
Change-Id: I5e7e6f0323b2e6145dc25e2b8e99b77557f4ce13

libs/WindowManager/Shell/tests/flicker/src/com/android/wm/shell/flicker/pip/PipKeyboardTest.kt

@@ -76,12 +76,12 @@ class PipKeyboardTest(
             }
             teardown {
                 test {
-                    keyboardApp.exit()
+                    keyboardApp.forceStop()
 
                     if (device.hasPipWindow()) {
                         device.closePipWindow()
                     }
-                    testApp.exit()
+                    testApp.forceStop()
                     this.setRotation(Surface.ROTATION_0)
                 }
             }