Attempt to fix some recent flakiness in testAwaitFullGc.

I'm doing so by following the advice from martinrb@ in cl/26588812.

I don't really have a good way to test this, but presumably I'll notice eventually if the flakiness remains.

RELNOTES=n/a
PiperOrigin-RevId: 716747770

Author: cpovirk

android/guava-testlib/test/com/google/common/testing/GcFinalizationTest.java

@@ -233,8 +233,8 @@ protected void finalize() {
     // Use e.g. awaitClear or await(CountDownLatch) instead.
     GcFinalization.awaitFullGc();
 
-    // If this test turns out to be flaky, add a second call to awaitFullGc()
-    // GcFinalization.awaitFullGc();
+    // Attempt to help with some flakiness that we've seen: b/387521512.
+    GcFinalization.awaitFullGc();
 
     assertEquals(0, finalizerRan.getCount());
     assertNull(ref.get());

guava-testlib/test/com/google/common/testing/GcFinalizationTest.java

@@ -233,8 +233,8 @@ protected void finalize() {
     // Use e.g. awaitClear or await(CountDownLatch) instead.
     GcFinalization.awaitFullGc();
 
-    // If this test turns out to be flaky, add a second call to awaitFullGc()
-    // GcFinalization.awaitFullGc();
+    // Attempt to help with some flakiness that we've seen: b/387521512.
+    GcFinalization.awaitFullGc();
 
     assertEquals(0, finalizerRan.getCount());
     assertNull(ref.get());