Prepare for 8.2 bugfix release.
Update to revision 5 of Play billing.

- Security fixes.
- Fix for disposed crashes.
Insert recommended null checks for IabHelper, use global debug flag.

- Check for a null IabHelper in more places as recommended by the
TrivialDrive IAB sample.
- Use the global BuildConfig.DEBUG flag instead of a local one.
Update change log.
Disable adding account from Android settings.
Update change log.
Merge branch 'fix-8.2'