Remove spurious error logging from BackgroundDexOptService

The old logic created a log entry indicating an error when the
actual result was success.

Make the log entry accurate.

Also, ensure that an out-of-space failure is actually reported.

Fixes: 136201068
Test: Manually checked 'adb logcat'
Change-Id: I52110b84496d9fc90bf08d16cb434ea0c979fa04