Set $TMPDIR as well as java.io.tmpdir.

This helps functions like tmpnam() and tmpfile() to "just work", like
their Java equivalents.

Bug: https://issuetracker.google.com/36991167
Bug: http://b/174682340
Test: atest FrameworksCoreTests:android.app.activity.ActivityThreadTest
Change-Id: Ife785f0ca69a34ba2c6b5d1e8e23f8b883d8fc8f

core/java/android/app/ActivityThread.java

@@ -6299,7 +6299,13 @@ public final class ActivityThread extends ClientTransactionHandler {
             final File cacheDir = context.getCacheDir();
             if (cacheDir != null) {
                 // Provide a usable directory for temporary files
-                System.setProperty("java.io.tmpdir", cacheDir.getAbsolutePath());
+                String tmpdir = cacheDir.getAbsolutePath();
+                System.setProperty("java.io.tmpdir", tmpdir);
+                try {
+                    android.system.Os.setenv("TMPDIR", tmpdir, true);
+                } catch (ErrnoException ex) {
+                    Log.w(TAG, "Unable to initialize $TMPDIR", ex);
+                }
             } else {
                 Log.v(TAG, "Unable to initialize \"java.io.tmpdir\" property "
                         + "due to missing cache directory");

core/tests/coretests/src/android/app/activity/ActivityThreadTest.java

@@ -87,6 +87,11 @@ public class ActivityThreadTest {
             new ActivityTestRule<>(TestActivity.class, true /* initialTouchMode */,
                     false /* launchActivity */);
 
+    @Test
+    public void testTemporaryDirectory() throws Exception {
+        assertEquals(System.getProperty("java.io.tmpdir"), System.getenv("TMPDIR"));
+    }
+
     @Test
     public void testDoubleRelaunch() throws Exception {
         final Activity activity = mActivityTestRule.launchActivity(new Intent());