Switch from Calculator to ExactCalculator

Bug: 24875598

Use ExactCalculator for AOSP builds.

This is a manual (due to intervening commits) cherrypick of
commit 53c5891460408ffe37f2aecc2044320d49cef801 (ag/748830).

Change-Id: I6c0acef640e62f8a70b7b2832b7eea662a0d301b
1 file changed