adb: win32: fix key files reading/writing

The issue is that adb uses fopen() with "e" (presumably to open the file
with O_CLOEXEC), but that flag causes MSVCRT.DLL to return an error. So
when adb_auth_host.cpp goes to read or write the adbkey files, it fails.

The quick fix is to not use the "e" option on adb host code since it
isn't necessary there, compared to adbd.

An alternative fix would be to have a fopen() wrapper on Windows that
filters out the "e" option.

Bug: http://b/21806456
Bug: https://code.google.com/p/android/issues/detail?id=175077


Change-Id: I7d8ba2847dab0ed558ffe156e79093251eb253c9
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>
(cherry picked from commit 9b960314)