Import mpdecimal-4.0.0

Third-Party Import of: https://www.bytereef.org/mpdecimal/
Request Document: go/android3p
For CL Reviewers: go/android3p#reviewing-a-cl
For Build Team: go/ab-third-party-imports
Bug: http://b/390479029
Security Questionnaire: http://b/390479029#comment1

Test: m libmpdec
Change-Id: I274650578c996fc7ccf454d1d3a913714d10ac4b
146 files changed