[Android] Stop enclosing keystore-related Ant parameters in quotes.

Let the subprocess module take care of separating and escaping each
parameter instead. Not doing that causes different errors depending on
the platform:

* On Windows, the double quotes will be part of the values passed to Ant
  and will thus not be found, or the values of things such as passwords
  will contain extra characters and fail validation.

* On Unix platforms (this was tested on Debian), entries with spaces
  would be truncated at the first space character.

Adding a test for the latter is easier, so an extra key was added to
xwalk-test.keystore with the following command, with key password 'xwalk
test':

  keytool -genkey -v -keystore xwalk-test.keystore -alias 'xwalk test'
          -keyalg RSA -keysize 2048 -validity 36525

BUG=XWALK-2377
[Android] Prefer binaries with extensions in make_apk.py.

When calling a binary such as Ant on Windows, we are currently
preferring the "ant" executable over "ant.bat", even though the former
is actually a shell script for Unix installations.

Change the preference order and check for binaries with extensions
first.

BUG=XWALK-2377
[Android] Pass full paths when invoking `android' and `ant'.

Since we invoke subprocess.Popen() with shell=False, we cannot rely on
PATH being parsed, nor can we assume the right extension (such as
".bat") will be added to the executable we want to run. This is normally
not a problem on Unix-based systems, but on Windows we have to use
explicitly "android.bat" and "ant.bat".

Fortunately enough, we already have a Which() function calls
AddExeExtensions(), does all the required path detection and adds any
extension found in PATHEXT if necessary, so use that to determine the
right "android" and "ant" binaries to be invoked by make_apk.

It is important to note it is AddExeExtensions() that matters here -- we
just use absolute paths with Which() because it filters through all
possible executable names.

BUG=XWALK-2377
Merge pull request #2423 from rakuco/make_apk-windows-fixes

[Android] Make make_apk work on Windows again.