Fix Paint#breakText(char[]..) JavaDoc

The JavaDoc says that if the "...count is negative then the
characters before index are measured in reverse order. This
allows for measuring the end of string."

However, the native code doesn't work its way backwards
from the given index when the count is negative. It always
passes Skia the substring starting from index and the
absolute value of count.

Its better to fix the JavaDoc rather than the behavior, since
this API is public and has been this way for some time. It
is also used by TextUtils.ellipsize indirectly, so its best
to avoid breaking that sensitive code.

Change-Id: I8bd51def6ec98251e355370710eb44f58fb737c4