Set STL explicitly for a test jni library

`jni_dep` is a jni dep of a test android app (named `app`) that runs on
CI. `app` sets an sdk_version, therefore we should build the sdk variant
of `jni_dep`.

stl.go (and stl.bzl) set different default values of stl for shared
libs depending on whether the lib is a platform or an sdk variant
- Platform variant: "libc++"
- Sdk variant: "" (will use libstdc++)
(https://cs.android.com/android/_/android/platform/build/soong/+/main:cc/stl.go;l=85-86,108-112;drc=72b8fcbbec330f2c9f0465a551be287fbe8101f7;bpv=1;bpt=0)

jni.cc uses `sstream` which is part of libc++ and not libstdc++. In
preparation for supporting sdk variants of jni libraries in b builds,
set the stl explicitly to `libc++`. This will compile jni.cc against the
NDK variant of libc++

Test: b build //build/bazel/... --config=android

Change-Id: I29147c776ca4d2fb60d4d0d139d63c8378743c04