Common testcase directories: Clean up duplicates

This CL addresses the problem that native tests build for multiple
architectures and also build for host.

First it restructures each testcase folder to the following layout:
<testcase>/<testcase>.config
<testcase>/<arch 1>/testcase
<testcase>/<arch 2>/testcase

Secondly it now uses the new soong-zip prefix capability to
separate out host and target testcases in the device-tests &
general-tests zip files.

Bug:36692141
Test: `make device-tests general-tests cts -j` &
      ensured all built successfully.

Change-Id: I4e8b084ee56fa8940914911649ff35507d87eb27