Store and pull files from /sdcard/

Tradefed FilePullerLogCollector can no longer access the internal app directory (e.g. /storage/emulated/0/Android/data/com.android.server.wm.flicker/files).

This makes it unable to pull trace files, when a test fails. This CL makes flicker tests store their data on /sdcard/flicker which can be accessed by the Tradefed FilePullerLogCollector

Bug: 171026227
Test: atest WMShellFlickerTests
Change-Id: Ie2329cbc475ee1be35e6bb3e274c96a4db142ac5