Set MigTextSpec As Focusable If Accessibility Heading is Enabled

Summary:
## Context
There is over 112 instances of MigTextSpec is set with `accessibilityHeading(true)` but without `screenReaderFocusable(true)` which can prevent talkback from annoucing the accessibility heading.

In this diff we are setting `screenReaderFocusable(true)` as the default if `accessibilityHeading(true)` is set.

Reviewed By: denley

Differential Revision: D69086861

fbshipit-source-id: 93b3971bdb1f2b55336f1043190b21c9cf729e93

litho-processor/src/main/java/com/facebook/litho/specmodels/model/PropValidation.java

@@ -136,6 +136,7 @@ public class PropValidation {
           "fullImpressionHandler",
           "invisibleHandler",
           "contentDescription",
+          "accessibilityHeading",
           "viewTag",
           "viewTags",
           "shadowElevationPx",
@@ -244,6 +245,7 @@ public class PropValidation {
                   ClassName.bestGuess("com.facebook.litho.EventHandler"),
                   ClassName.bestGuess("com.facebook.litho.InvisibleEvent"))),
           new CommonPropModel("contentDescription", ClassName.bestGuess("java.lang.CharSequence")),
+          new CommonPropModel("accessibilityHeading", TypeName.BOOLEAN),
           new CommonPropModel("viewTag", TypeName.OBJECT),
           new CommonPropModel("viewTags", ClassName.bestGuess("android.util.SparseArray")),
           new CommonPropModel("shadowElevationPx", TypeName.FLOAT),