adb/base: fix adb push of Unicode filenames on Win32

ae5a6c06 made adb push use
android::base::ReadFileToString() for small files, but that API did not
support UTF-8 filenames on Windows, until this fix which does the
following:

- Add android::base::{WideToUTF8,UTF8ToWide}() which are only available
  on Windows. The signatures are based on Chromium's APIs of the same
  name.

- Add the namespace android::base::utf8 which has versions of APIs that
  take UTF-8 strings. To use this, make sure your code is in a namespace
  and then do "using namespace android::base::utf8;". On Windows, this will
  make calls to open() call android::base::utf8::open(), and on other
  platforms, it will just call the regular ::open().

- Make ReadFileToString() and WriteStringToFile() use utf8::open() and
  utf8::unlink().

- Adapt unittests from Chromium.

- fastboot needs to link with libcutils because it links with libbase
  which depends on libcutils for gettid() for logging.

Change-Id: I1aeac40ff358331d7a1ff457ce894bfb17863904
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>