Don't delete the new file in openRead().

Although it may seems a left-over from a previous interrupted write,
actually there are callers who call startWrite(), openRead() and then
finishWrite(), and this was okay in the previous implementation, so we
have to keep supporting it.

The new file is virtually ignored in the new implementation, and we
have no good way to know if it's actually a left-over or one that's
being written, so simply leaving it there is also okay.

Fixes: 157092639
Test: atest AppIdleHistoryTests#testFilesCreation
Change-Id: I4dc7fde99d2b8e04356f082a6e6ad61c2835022e