Fix AccessibilityDumper screenshot path.

This screenshot path contained invalid ':' chars according
to FileUtils.java

Fix: 277365782
Test: Intentionally change a test to fail, observe failure screenshot
      is saved correctly.
Change-Id: Ic0a9fe4c4edcee534edf23e581dd367b52239133
diff --git a/tests/accessibility/common/src/android/accessibility/cts/common/AccessibilityDumper.java b/tests/accessibility/common/src/android/accessibility/cts/common/AccessibilityDumper.java
index 8d631d4..bbd63c7 100644
--- a/tests/accessibility/common/src/android/accessibility/cts/common/AccessibilityDumper.java
+++ b/tests/accessibility/common/src/android/accessibility/cts/common/AccessibilityDumper.java
@@ -153,7 +153,8 @@
     private void dumpScreen(UiAutomation automation) {
         assertNotEmpty(mName);
         final Bitmap screenshot = automation.takeScreenshot();
-        final String filename = String.format("%s_%s__screenshot.png", mName, LocalTime.now());
+        final String filename = String.format("%s_%s__screenshot.png", mName,
+                LocalTime.now().toString().replace(':', '.'));
         BitmapUtils.saveBitmap(screenshot, mRoot.toString(), filename);
     }