Make TextView.isSingleLine public

Bug: 117521096
Test: Added new CTS tests under cts.TextViewTest
Test: atest CtsWidgetTestCases:android.widget.cts.TextViewTest
Change-Id: Ib19277767c69fb2bf92d2387738dbb228f31f502

api/current.txt

@@ -55578,6 +55578,7 @@ package android.widget {
     method public boolean isElegantTextHeight();
     method public boolean isFallbackLineSpacing();
     method public boolean isInputMethodTarget();
+    method public boolean isSingleLine();
     method public boolean isSuggestionsEnabled();
     method public boolean isTextSelectable();
     method public int length();

core/java/android/widget/TextView.java

@@ -6254,8 +6254,13 @@ public class TextView extends View implements ViewTreeObserver.OnPreDrawListener
         return mHint;
     }
 
-    @UnsupportedAppUsage
-    boolean isSingleLine() {
+    /**
+     * Returns if the text is constrained to a single horizontally scrolling line ignoring new
+     * line characters instead of letting it wrap onto multiple lines.
+     *
+     * @attr ref android.R.styleable#TextView_singleLine
+     */
+    public boolean isSingleLine() {
         return mSingleLine;
     }