Fix string patch target retrieval in OatWriter.

Use the correct dex cache for retrieving the target string
for const-string inlined across dex files.

This affects only the boot image built in non-debuggable
mode because we do not inline at all in debuggable mode.
No regression test is provided because we do not have the
infrastructure to compile test code into the boot image.

Change-Id: I203419570a22ad78f30a385628ac6b57aea5bec1