Fix rotation test on tablets

Although we set "android:configChanges="orientation|screenSize" for TestActivity, it is still recreated after rotation on tablets. After recreation, the old activity and EditText view are dropped, which causes the test failures. Fix this by:
1. Remove android:configChanges to trigger recreation at all devices
2. Record the last created instance of TestActivity to get the new activity
3. Verify the show/hide behavior for the new acitivity. Note that the keyboard will not show after activity recreation only when visibility flag is SOFT_INPUT_STATE_ALWAYS_HIDDEN.

Bug: 275666243
Test: atest com.android.inputmethod.stresstest.ImeOpenCloseStressTest#testRotateScreenWithKeyboardOn
Change-Id: I113c0b1376febed5cb6dace9cd0467339f77c699