ART: Add FdFile::Copy() to copy data from another file.

Also move utilities for inspecting file magic numbers to
base/file_magic.{h,cc} and drop the unused IsOatMagic().

Change-Id: I2cc4dd18a5e8b9738fb386c8057faad3722bdd68
diff --git a/runtime/Android.mk b/runtime/Android.mk
index 571a2f5..74cc899 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -27,6 +27,7 @@
   base/arena_allocator.cc \
   base/arena_bit_vector.cc \
   base/bit_vector.cc \
+  base/file_magic.cc \
   base/hex_dump.cc \
   base/logging.cc \
   base/mutex.cc \