Fix clang static analyzer warning in libcore

There are two potential memory leaks in
libcore/luni/src/main/native/libcore_icu_NativeConverter.cpp. The
callbackContext would not be deleted before returning if it was newly
created.

Fixed it by using an unique_ptr to manage it.

Bug: 63876067

Test: mmm with static analyzer and warning disappears. Ran CTS test for
CtsLibcoreTestCases and no new errors.

Change-Id: If700a31fe10065a67e4aa4044fa47672d1dd6a0d
1 file changed