Fix transaction capping bug.

We don't want to give out credits if that would result in there being
too many credits in circulation or the app accumulating too many
credits, so we cap the transaction delta to stay within the limits. We
were accidentally taking away credits because of the capping, so this
makes sure we don't do that.

Bug: 158300259
Test: atest FrameworksMockingServicesTests:AgentTest
Change-Id: Idc82af353d1021577ead2e0038145dfb8cdffeab