Fix BigDecimal result for -Long.MIN_VALUE / -1L

BigDecimal produced the result -9223372036854775808 (== -2^64
== Long.MIN_VALUE) instead of 9223372036854775808 (== 2^64).
This was because BigDecimal incorrectly used long arithmetic for
this calculation, which overflows back to Long.MIN_VALUE.

This bug didn't happen in the case where the result scale required
adjustment (diffScale != 0): The bitLength(power of ten) calculation
rounds up to whole bits, and therefore is generous enough not to fall
into this trap in the corner case.

Public bug: https://code.google.com/p/android/issues/detail?id=196555

Bug: 26105053
Change-Id: Ie4b1d08c9074d59d854c664ef17aedcaae24c56e