[Bold text] Use a default bold font for null typefaces

When a TextView's typeface is null and bold is enabled, Typeface.DEFAULT
is used. Instead, this should be Typeface.DEFAULT_BOLD

Flag:
adb shell device_config put text com.android.text.flags.fix_null_typeface_bolding true

Bug: 314811487
Test: manual, CTS
Change-Id: I1be9d6c17b6736caa28e280872bf4b1dac4c7855

core/java/android/text/flags/flags.aconfig

@@ -201,3 +201,13 @@ flag {
     purpose: PURPOSE_BUGFIX
   }
 }
+
+flag {
+  name: "fix_null_typeface_bolding"
+  namespace: "text"
+  description: "Use a bold typeface when bolding is enabled and the original typeface is null"
+  bug: "314811487"
+  metadata {
+    purpose: PURPOSE_BUGFIX
+  }
+}

core/java/android/widget/TextView.java

@@ -4817,7 +4817,11 @@ public class TextView extends View implements ViewTreeObserver.OnPreDrawListener
         if (mFontWeightAdjustment != 0
                 && mFontWeightAdjustment != Configuration.FONT_WEIGHT_ADJUSTMENT_UNDEFINED) {
             if (tf == null) {
-                tf = Typeface.DEFAULT;
+                if (Flags.fixNullTypefaceBolding()) {
+                    tf = Typeface.DEFAULT_BOLD;
+                } else {
+                    tf = Typeface.DEFAULT;
+                }
             } else {
                 int newWeight = Math.min(
                         Math.max(tf.getWeight() + mFontWeightAdjustment, FontStyle.FONT_WEIGHT_MIN),