Use /sdcard instead of /data for cached version files

The test app can't access /data, so /sdcard is a better place to place
these files, since it will work for any test as opposed to using
/data/data/com.android.functional.otatests.

Bug: 28222612
Change-Id: I09eb4944cc8b1bb0457ee776305412f9b6f95c72