Fixing android.text.format.Time for non-English locales

The test fixes for bug 17262063 showed up a real issue for
non-English locales with the Time.format() method:
If the Android string resources that contain the pattern use
non-ASCII characters then a '?' would be output instead of
those characters.

For example, in France the pattern for '%c' includes a 'à'
(a with a grave accent) and Japan includes 日.

The problem was due to converting the pattern to bytes using
the US_ASCII character set, which turns non-ASCII characters
into '?'. The code has been changed to use char throughout
and avoid bytes.

Internal documentation has been improved.

Some calls to modifyAndAppend() have been replaced with a
direct call to outputBuilder.append() because the
modify step is guaranteed to a no-op for the literals given.

The formatter has been changed to use Locale.US because it
is only used for outputting numbers. It has been renamed
to make this more obvious and the locale field has been
removed.

Bug: 17262063
Change-Id: I32b92f7f7e3e6931d3514d87f1d9a38f136d4021