Explicit cast from android::Base::Errno to status_t

Previously, Result<T>.error().code() returned int and this was
convertible to status_t which is an alias of int.

However, this actually was type unsafe because the return type of
error().code() was actually mean to be that of errno. errno and
status_t are both int, but their meanings are very different.

To handle such problems, Result<T>.error().code() now returns
android::base::Errno which is a wrapper to errno, but not to other
int-based types like status_t and StatusCode.

Eventually, Result<T> used here should be changed into Result<T,
StatusT> where StatusT is a type-safe wrapper class to status_t that
implements the contract to be used as the error type in
android::base::Result.

Unfortunately, since the use of Result<T> is so wide spread, in this
change, I aim to only fix the build error in this CL.

Bug: 209929099
Test: m
Change-Id: Ic10ab8ed52f6827ba3ac6fb7d6e031585827562f