Modify isShowing method in Dialog

The method isShowing in Dialog.java is to judge whether
the dialog is currently showing, but it will return true
when the dialog is hidden.

Bug: 74416088
Test: modify testShow in DialogTest.java (see 639879)
run cts -m CtsAppTestCases -t android.app.cts.DialogTest#testShow

Change-Id: Ibf15e080678f414f8a66ff0841be17a2f9d2d652
Signed-off-by: tiansiming [田思明] <tiansiming@xiaomi.com>