Move apksigner library to tools/apksig.

This moves build/tools/apksigner/core to its own project tools/apksig.
The move also renames the moved Java packages from
com.android.apksigner.core.* to com.android.apksig.* to reflect the
new name of the library.

Bug: 27461702
Change-Id: Iab812ae2b8f0a741014f842460c78e35bc249d43