aconfig: include all tests in TEST_MAPPING

The aconfig tests used to be implicitly run by Treehugger, but something
has changed and Treehugger no longer verifies aconfig uploads. Fix this
by explicitly listing all aconfig tests in the TEST_MAPPING file.

Treehugger does not allow new tests in presubmit before they have proven
themselves (in terms of flakiness and execution speed) in postsubmit.
For this reason this CL adds the tests to postsubmit; a follow-up CL
will move them to presubmit.

This has the added benefit of allowing developers to easily run all
tests locally, either by

  $ atest

if current working directory is build/tools/aconfig or one of its
subdirectories, or

  $ atest --test-mapping $(gettop)/build/tools/aconfig

from anywhere in the Android tree.

Also add all tests to "general-tests" to enable Treehugger to run them.
Move aconfig.test from "device-tests" to "general-tests"; the former
group is intended for tests that depend on device-specific
functionality. See [1] for more info.

1. https://source.android.com/docs/core/tests/development/test-mapping

Bug: N/A
Test: atest --test-mapping $(gettop)/build/tools/aconfig
Change-Id: If857733834c8ad43a16e6162e50e6b1f713f979d