OpenJDK 11: Merging in java.math.* trivial changes

Updated MutableBigInteger, MathContext, RoundingMode and
package-info.

Added tests for RoundingMode constants.

New package private APIs in MutableBitIngeter are covered with unit
tests in the subsequent CL as it is used for BigInteger.sqrt() and
BigDecimal.sqrt().

Bug: 200171748
Test: m droid
Change-Id: I12b3f599a16c3c904c4d8d4ac049eac59a735ca3
5 files changed