Android: Speed up incremental install test listing by looking only in test dexes

On my machine, this sped up the test listing step by 50% according to
the timestamps from bin/run_chrome_public_test_apk_incremental -v.

While this is a heuristic, I tested that it finds all tests for
chrome_public_test_apk, and if it does ever miss some tests, it won't
affect bots, so the error will happen only locally.

Change-Id: If227a3b08963d8899839bd6d661f0f392ff1cd5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1777092Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692122}

base/test/android/javatests/src/org/chromium/base/test/BaseChromiumAndroidJUnitRunner.java

@@ -31,6 +31,7 @@ import org.chromium.base.annotations.MainDex;
 import org.chromium.base.multidex.ChromiumMultiDexInstaller;
 import org.chromium.base.test.util.InMemorySharedPreferencesContext;
 
+import java.io.File;
 import java.io.IOException;
 import java.lang.reflect.Field;
 import java.util.ArrayList;
@@ -213,12 +214,25 @@ public class BaseChromiumAndroidJUnitRunner extends AndroidJUnitRunner {
     }
 
     private TestRequest createListTestRequest(Bundle arguments) {
-        List<DexFile> dexFiles = new ArrayList<>();
+        ArrayList<DexFile> dexFiles = new ArrayList<>();
         try {
             Class<?> bootstrapClass =
                     Class.forName("org.chromium.incrementalinstall.BootstrapApplication");
-            dexFiles = Arrays.asList(
-                    (DexFile[]) bootstrapClass.getDeclaredField("sIncrementalDexFiles").get(null));
+            DexFile[] incrementalInstallDexes =
+                    (DexFile[]) bootstrapClass.getDeclaredField("sIncrementalDexFiles").get(null);
+            // The incremental install dex files contain directory paths in their names. e.g.:
+            //   third_party.android_deps.com_android_support_gridlayout_v7_java__classes.dex.jar
+            // Skipping application and system dex files speeds up test listing 12s -> 6s on my
+            // Pixel 1 running Q.
+            for (DexFile dexFile : incrementalInstallDexes) {
+                if (dexFile.getName().startsWith("/system")) {
+                    continue;
+                }
+                String baseName = new File(dexFile.getName()).getName();
+                if (baseName.contains("test") || baseName.contains("Test")) {
+                    dexFiles.add(dexFile);
+                }
+            }
         } catch (Exception e) {
             // Not an incremental apk.
             if (BuildConfig.IS_MULTIDEX_ENABLED