ANDROID: Add kunit targets.

The kunit targets build in-tree kunit modules with a pre-set
defconfig fragment.

This is the defconfig fragment to be applied to enable kunit tests
as modules. Because we apply defconfig fragments after running
make defconfig, the individual tests must be enabled instead of
a global CONFIG_KUNIT_ALL_TESTS flag.

Test: manually build with
  bazel run //common:kunit_aarch64_dist
  bazel run //common:kunit_x86_64_dist
  bazel run //common:kunit_riscv64_dist
Bug: 296116800
Change-Id: If3b94f495aa64b657508781b12e3e0716c725f3f
Signed-off-by: Yifan Hong <elsk@google.com>