binderStabilityTest: rewrite pieces w/ no AIDL

Since AIDL sets interface stability now, we can't use AIDL interfaces in
this test. A custom C++/NDK interface has instead been written there so
that we can set different stability levels in the same compilation unit.
Writing custom interfaces like this is strongly discouraged and is not
supported. However, in order to test different stability levels in the
same module, we need to be able to do this, and android testing
infrastructure doesn't easily support running things in multiple
compilation contexts yet.

Bug: 138467287
Test: binderStabilityTest
Change-Id: I4461b82baf4c9850cfb4c32d7b0c5ca82fa83a74