Prefix temp files to ensure they are cleaned up

Temp files are removed unconditionally in a finally block following extraction.
However, if the process is killed during extraction this finally block will not
run. Because temp files started with extractedFilePrefix, they wouldn't be
cleaned up in prepareDexDir(). This change ensures that prepareDexDir() will
remove any existing temp files before extraction begins.

Bug: 27769642
Bug: 33718827
Test: Passes Google Play services tests
Change-Id: I803ba2c7234801551d36cbbe2941eeaa986d31f8