Update Transaction for boot image extension.

And clean up transaction-related code to keep test code
out of the production binaries.

Test: Add TransactionTest#Constraints to transaction_test.
Test: m test-art-host-gtest
Test: testrunner.py --host
Test: aosp_taimen-userdebug boots.
Change-Id: Iefe5f1cfde95f564069249148f9e7d71564d7a10
12 files changed