androidfw: Use the last package to create idmap.

Overlaid packages contain one or two package groups, the first one is
system package(android), and the second one is application package. So
it should use the last package group to create idmap.

When inspecting the idmap, similar as above.

Change-Id: I97e0179bc5b5acdcacbe6ec5b5b7c22af51d1d0b