adb: win32: remove widen()/narrow() in favor of UTF8ToWide()/WideToUTF8()

Now that we have a more standardized API (also available in Chromium),
switch to it. Another benefit is real error handling instead of just
killing the process on invalid Unicode.

Make UTF8ToWide()/WideToUTF8() set errno to EILSEQ on bad input. This is
the same error code that wcsrtombs(3) uses.

Update the unittest to check for EILSEQ.

Change-Id: Ie92acf74d37adaea116cf610c1bf8cd433741e16
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>