Include missing mutex.h for Android

PiperOrigin-RevId: 493676427

internal/platform/BUILD

@@ -257,6 +257,7 @@ cc_test(
         "//internal/platform/implementation/g3",  # build_cleaner: keep
         "@com_github_protobuf_matchers//protobuf-matchers",
         "@com_google_absl//absl/hash:hash_testing",
+        "@com_google_absl//absl/synchronization",
         "@com_google_googletest//:gtest_main",
     ],
 )

internal/platform/prng_test.cc

@@ -20,6 +20,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/synchronization/mutex.h"
 
 namespace location {
 namespace nearby {