WebRTC: Finish setting up for the migration to JNI Zero's DEFINE_JNI()

JNI Zero is launching a new feature that requires all .cc files to
add a macro call DEFINE_JNI(JavaClassName). This CL does two things:
1. Add DEFINE_JNI() to two files in the examples directory.
I missed these two files in my previous CL
https://webrtc-review.googlesource.com/c/src/+/489720

2. Stop using @JniType in NativeTestWebrtc.java. This is the only
usage of @JniType in WebRTC, and stop using @JniType in WebRTC
would make the migration a lot easier.

To be clear, the @JniType syntax is still fully supported, and you
can feel free to use it once we finish the migration.
But for the purpose of this migration, getting rid of the only usage
of it would make the migration a lot easier, hence we converted it
to an equivalent, but more verbose, syntax.

Bug: chromium:481689330
Change-Id: Ia2a67c12580f449ab05289216ff09fabbda0c7e5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/490900
Commit-Queue: Martin Kong <martinkong@google.com>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48226}