Prepare to make ImmutableSet.toImmutableSet available in guava-andr…

…oid (but don't do so yet).

This includes adding `ImmutableCollection.spliterator()` as a further attempt to shake out any problems that might arise when we make "real" API changes. The method is callable under API Level 24 or higher, under which it overrides the method inherited from `Collection`. (But when last I checked (over 2 years ago!), library desugaring rewrote it to _not_ be an override: b/187166349.) In any case, we don't expect users to call it directly.

(Compare cl/543468006.)

This CL also includes updates to avoid problems with reflection when Java 8+ APIs are not present. That means adding a `serialVersionUID` (so that Java/Android reflection doesn't try to compute one with reflection) and skipping `NullPointerTester` under Android (though we still run `NullPointerTester` against the Android flavor under a JVM). (I might have added `serialVersionUID` more places than necessary; I'm not sure.)

(Compare cl/550872250.)

This CL is further progress toward #6567

(To make this work with some of our Android library-desugaring infrastructure, I had to first implement the new `SequencedCollection` methods in `ImmutableSortedSet`. That happened in cl/575207552.)

RELNOTES=n/a
PiperOrigin-RevId: 573288649

android/guava-tests/test/com/google/common/collect/FauxveridesTest.java

@@ -61,10 +61,12 @@ public void testImmutableSortedMap() {
     doHasAllFauxveridesTest(ImmutableSortedMap.class, ImmutableMap.class);
   }
 
+  @AndroidIncompatible // similar to ImmutableTableTest.testNullPointerInstance
   public void testImmutableSortedSet() {
     doHasAllFauxveridesTest(ImmutableSortedSet.class, ImmutableSet.class);
   }
 
+  @AndroidIncompatible // similar to ImmutableTableTest.testNullPointerInstance
   public void testImmutableSortedMultiset() {
     doHasAllFauxveridesTest(ImmutableSortedMultiset.class, ImmutableMultiset.class);
   }

android/guava-tests/test/com/google/common/collect/ImmutableListTest.java

@@ -462,6 +462,7 @@ public void testToImmutableList_java7_combine() {
   // Basic tests
 
   @GwtIncompatible // NullPointerTester
+  @AndroidIncompatible // see ImmutableTableTest.testNullPointerInstance
   public void testNullPointers() {
     NullPointerTester tester = new NullPointerTester();
     tester.testAllPublicStaticMethods(ImmutableList.class);

android/guava-tests/test/com/google/common/collect/ImmutableMultisetTest.java

@@ -471,6 +471,7 @@ public void testBuilderSetCountIllegal() {
   }
 
   @GwtIncompatible // NullPointerTester
+  @AndroidIncompatible // see ImmutableTableTest.testNullPointerInstance
   public void testNullPointers() {
     NullPointerTester tester = new NullPointerTester();
     tester.testAllPublicStaticMethods(ImmutableMultiset.class);

android/guava-tests/test/com/google/common/collect/ImmutableSortedMultisetTest.java

@@ -431,6 +431,7 @@ public void testBuilderSetCountIllegal() {
     assertThrows(IllegalArgumentException.class, () -> builder.setCount("a", -2));
   }
 
+  @AndroidIncompatible // see ImmutableTableTest.testNullPointerInstance
   public void testNullPointers() {
     new NullPointerTester().testAllPublicStaticMethods(ImmutableSortedMultiset.class);
   }

android/guava-tests/test/com/google/common/collect/ImmutableSortedSetTest.java

@@ -230,6 +230,7 @@ protected <E extends Comparable<? super E>> SortedSet<E> copyOf(Iterator<? exten
   }
 
   @GwtIncompatible // NullPointerTester
+  @AndroidIncompatible // see ImmutableTableTest.testNullPointerInstance
   public void testNullPointers() {
     new NullPointerTester().testAllPublicStaticMethods(ImmutableSortedSet.class);
   }

android/guava-tests/test/com/google/common/collect/IterablesTest.java

@@ -1396,6 +1396,7 @@ public void testMergeSorted_skipping_pyramid() {
   }
 
   @GwtIncompatible // reflection
+  @AndroidIncompatible // see ImmutableTableTest.testNullPointerInstance
   public void testIterables_nullCheck() throws Exception {
     new ClassSanityTester()
         .forAllPublicStaticMethods(Iterables.class)