Use AssertionError(String, Throwable) instead of supplying the cause later.

I could have done this as part of cl/609415936 and cl/604677493, but I'd missed that `AssertionError(String, Throwable)` was [added in API Level 19](https://developer.android.com/reference/java/lang/AssertionError#AssertionError(java.lang.String,%20java.lang.Throwable)).

(The code in `Platform` in cl/609415936 still needs to use `initCause` because it uses `ComparisonFailure`, which does not expose a `Throwable`-accepting constructor.)

RELNOTES=n/a
PiperOrigin-RevId: 638335715

Author: cpovirk

core/src/main/java/com/google/common/truth/AssertionErrorWithFacts.java

@@ -31,10 +31,8 @@ final class AssertionErrorWithFacts extends AssertionError implements ErrorWithF
 
   AssertionErrorWithFacts(
       ImmutableList<String> messages, ImmutableList<Fact> facts, @Nullable Throwable cause) {
-    super(makeMessage(messages, facts));
+    super(makeMessage(messages, facts), cause);
     this.facts = checkNotNull(facts);
-
-    initCause(cause);
   }
 
   @Override

core/src/main/java/com/google/common/truth/Truth.java

@@ -355,9 +355,7 @@ public static PathSubject assertThat(@Nullable Path actual) {
   @SuppressWarnings("OverrideThrowableToString") // We intentionally replace the normal format.
   static final class SimpleAssertionError extends AssertionError {
     private SimpleAssertionError(String message, @Nullable Throwable cause) {
-      super(checkNotNull(message));
-
-      initCause(cause);
+      super(checkNotNull(message), cause);
     }
 
     static SimpleAssertionError create(String message, @Nullable Throwable cause) {