Remove the cache from libcore.icu.DateIntervalFormat

The class is not used by @UnsupportedAppUsage, and performance is no
longer a major concern for such usage. And app can use ICU4J directly,
and cache the neccessary formatter to fix such issue.

Also, mark some methods package private to restrict visibility

Bug: 160606356
Test: atest CtsLibcoreTestCases:libcore.libcore.icu.DateIntervalFormatTest
Change-Id: I9d68e1f90a8925707827917da011264f045282b8
2 files changed