Reserve space in the char backing vector to prevent reallocations

The pointers in image_filenames_ and oat_filenames can become invalidated
if the char_backing_storage_ vector gets reallocated when adding strings.

Test: mma test-art-host-gtest-dex2oat_test
Change-Id: I82714468163ce49ce53953e6edcb57a17d46cdbb
1 file changed