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

Merged-In: I2571ecc9e561294033ff13460bdd363a0ffc6c9b
(cherry picked from commit e6e9308d)
Change-Id: I80d2619e945e415e6ad8c5966a73f44e8dc5b6de