Move common apishim classes to apishim.common pkg

This provides a better separation of the classes based on their role (an
actual shim, or an interface or utility common to all shims), and forces
the apishim package to import the common classes, which makes it easier
at SDK bump time since the imports do not need to be added at that
point.

Bug: 143586229
Test: atest NetworkStackTests
Original-Change: https://android-review.googlesource.com/1310517
Merged-In: Ie77c17a96a67016f35890afa065bea0a07b09504
Change-Id: Ie77c17a96a67016f35890afa065bea0a07b09504