Eliminate synthetic access to invoke0()

This method is invoked from InvocationStrategy, hence it being
private generates a synthetic accessor method. Change method
visibility to package-private, eliminating this inefficiency.

Change-Id: I74ee9f8c729ba8d0892316fc405a0d99830b5f04
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>