ART: Unlink target oat file before compiling

Dex2oat must create a new file (new inode) when writing to a target
given by name, as the existing file may be in use. So unlink any
existing file first.

Bug: 22047255

(cherry picked from commit 52f0aeb8e9f16fa5c3067f6a36be701354448924)

Change-Id: Ief942c71564076e39c1e8340d5c4c55286f75896
1 file changed