Move JetPlayer implemention and JNI

The JetPlayer implementation in libmedia was only used by the
JetPlayer JNI code in libandroid_runtime. This change moves
both implementation and JNI to libmedia_jni. This reduces
libandroid_runtime's dependency on libmedia, and results in
a net size reduction of the libraries involved.

Test: atest JetPlayerTest
Change-Id: I028c774fdea167924b064855267254c15f22fddb