Fix CurrencyTest.test_getSymbol for unknown variants

The behavior of CurrencyTest.getSymbol has changed since
K for unrecoginzed locales.

Commit de0eb683 now means
the ICU default locale is set when the Java default locale
is changed. Previously it would have been left unset and
would have fallen back to using the system settings (most
likely US or UK English during tests).

The test is setting the Java default locale to "kr-KR"
(a locale unrecognized by ICU) and requesting the currency
symbol for USD for locale "kr-KR" (still unrecognized by
ICU).

Before the commit: Because the kr-KR argument is unrecognized
ICU tries the ICU default (which was unset and so picked
system setting of en). It returned "$".

After the commit: As above, but now the ICU default is set to
kr-KR. Since there is no entry for USD in the default it falls
back to "root", and returns the value found there: "US$".

Behavior when the default locale is unrecognized is edge-case
and arguably it is no more / less correct before or after
this commit. The test has been updated to reflect
the new value + some tidy up of the test generally.

This change also contains a more focused test that demonstrates
the behavior in ICU and adds documentation to make ICU
slightly clearer.

Arguably neither the new ICUTest nor the existing
CurrencyTest code should exist because they assert edge-case behavior
that Android may wish to change in future.

Bug: 14821275
Change-Id: I8beb6004b0c37d2fe59855d13d2c8e2e6fe1c530