adb: win32: define UNICODE/_UNICODE, Unicode error messages, misc

Define the UNICODE and _UNICODE preprocessor symbols to make passing
char* to Ansi/Unicode-agnostic Windows and C Runtime APIs break the
build. The solution is to call wide Windows and C Runtime APIs and use
widen(utf8).c_str(). Most code was already calling wide APIs. Defining
these symbols makes a call to CreateEvent() (which previously mapped to
CreateEventA()) turn into a call to CreateEventW().

Make SystemErrorCodeToString() use Unicode.

Add various comments.

Change-Id: I9b212412348a29826718e897a486489e1f142d16
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>