Fix warnings in TransactionAdapter

Fix warnings in TransactionAdapter

Using a private constructor forces a synthetic accessor being
created, hence make nested class constructors package-private,
eliminating the need to suppress warnings.

Also rename generic arguments so as not to clash with method
generics used in the interface being implemented.

Finally add proper generic arguments, so we eliminate raw type
warnings.

Change-Id: I9ec2bf6b377eed26fd11896e84c99fa8922f4abf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>