Change default output directory to use file dir.

It is not quite necessary to specify the external cache as the output
directory per discussion with bennolin@ months ago. For instrument
tests, we could just leverage the context's file dir.

Test: Unit test and http://ab/I82300010077210822
Change-Id: I4c648da095eb2671a7b8667897818683edf0196c
diff --git a/libraries/screenshot/src/main/java/platform/test/screenshot/GoldenImagePathManager.kt b/libraries/screenshot/src/main/java/platform/test/screenshot/GoldenImagePathManager.kt
index 72e1d4b..3d76912 100644
--- a/libraries/screenshot/src/main/java/platform/test/screenshot/GoldenImagePathManager.kt
+++ b/libraries/screenshot/src/main/java/platform/test/screenshot/GoldenImagePathManager.kt
@@ -167,7 +167,7 @@
  * Default output directory where all images generated as part of the test are stored.
  */
 public fun getDeviceOutputDirectory(context: Context) =
-    File(context.externalCacheDir, "androidx_screenshots").toString()
+    File(context.filesDir, "platform_screenshots").toString()
 
 /* Standard implementations for the usual list of dimensions that affect a golden image. */
 public fun getDeviceModel(): String {