Use Android Q compatible external dir

We previously used Environment.getExternalDir to obtain the output dir for the tests. However, since Android Q this method is deprecated and returns an invalid path, leading to errors. Replace this method for context.getExternalDir(null) that return a Q  friendly path.

Test: atest FlickerTests
Bug: 161341377
Change-Id: I5389ac6116b2b74fb19417683dbcfca604ca5b58
diff --git a/tests/FlickerTests/test-apps/flickerapp/AndroidManifest.xml b/tests/FlickerTests/test-apps/flickerapp/AndroidManifest.xml
index 7770c73..4d21440 100644
--- a/tests/FlickerTests/test-apps/flickerapp/AndroidManifest.xml
+++ b/tests/FlickerTests/test-apps/flickerapp/AndroidManifest.xml
@@ -17,8 +17,8 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.android.server.wm.flicker.testapp">
 
-    <uses-sdk android:minSdkVersion="17"
-         android:targetSdkVersion="27"/>
+    <uses-sdk android:minSdkVersion="29"
+         android:targetSdkVersion="29"/>
     <application android:allowBackup="false"
          android:supportsRtl="true">
         <activity android:name=".SimpleActivity"