Test for whitespace behavior in DecimalFormat.

This adds a number of tests that check how the space character is
treated in DecimalFormat.

These are not meant to be an unmoving specification that can never be
changed, but to act as a change detection test. If the behavior of the
underlying classes changes in a way that is considered reasonable, these
tests should be updated accordingly.

Test: bit CtsLibcoreTestCases:libcore.java.text.DecimalFormatTest
Bug: 68143370
Change-Id: I22174a4fa32af06dd349c1bccf26c3fbaebb26c0