Fix BigInteger test failures.

Bug: 2943474
Change-Id: Ia06bcf3025f525a954d9b918669d09531631a266

Conflicts:
	harmony-tests/src/test/java/org/apache/harmony/tests/java/math/BigIntegerConstructorsTest.java
	harmony-tests/src/test/java/tests/api/java/math/BigIntegerTest.java