Fix Matcher.find and reset

- Matcher.find didn't call reset(), but according to the javadoc, it
should.
- reset() didn't obtain a fresh value from mutable CharSequence.
  Given that the constructor uses CharSequence, it should obtain a new value
  from the CharSequence and match it again

Added 2 tests to verify such behaviors accordingly

Bug: 33737213
Test: cts-tradefed run cts -m CtsLibcoreTestCases --primary-abi-only
Change-Id: I16ce01944b8c1baac9b7947d5551fb030eb95fc8