libadf: adf_test: fix clang + kati build

kati needs us to add "tests" as a subdir of the libadf Android.bp.
clang also requires an explicit definition of AdfTest::dev_id where
g++ does not.

Test: mmm system/core/adf/libadf
Test: /data/nativetest64/adf-unit-tests/adf-unit-tests (on Nexus 9)
Change-Id: I1fc371af1b5b115f088edd963a87e6647a52882b
Signed-off-by: Greg Hackmann <ghackmann@google.com>