Remove unnecessary ConfiguredJarList.apex(int) method

This is only used from within the ConfiguredJarList implementation and
provides no real benefit over directly accessing the apexes slices.
Similarly, uses of Jar(int) from within the implementation are also
replaced with direct slice access.

Bug: 171479578
Test: m nothing
Change-Id: I7e799b1049f4a1da4140e55831c4559751278de6