Don't send AccessibilityEent if text is unchanged

It always sends TYPE_WINDOW_CONTENT_CHANGED event with
CONTENT_CHANGE_TYPE_TEXT when invoking the API TextView#setText.
However, performs action ACTION_SET_SELECTION will call the API and
send the event even though the text unchanged. So the fix tried to
send event only when text is changed.

Bug: 176707630
Test: atest com.android.internal.accessibility.AccessibilityUtilsTest
and manually test by performing action ACTION_SET_SELECTION

Change-Id: I16973f8648270cb0c652c2663615aed1435b0e2e