Expose API android.icu.util.IllformedLocaleException

android.icu.util.IllformedLocaleException is referenced in javadoc, but
is not exposed as a public API

Test: Existing tests cover this API. cts-tradefed run cts-dev
-m CtsIcuTestCases -t android.icu.dev.test.util.ULocaleTest
Test: m
Bug:71629543

Change-Id: I2571ecc9e561294033ff13460bdd363a0ffc6c9b