Remove AndroidRuntime::getJNIEnv() usage from libmedia2_jni

JavaVMHelper keeps the vm instance supplied with JNI_OnLoad and
it is reused whenever JniEnv is requested.
Plus, AndroidRuntime::registerNativeMethods() is replaced with
jniRegisterNativeMethods() in libnativehelper.

Test: pass MediaPlayer2Test#testPlayVideo
Bug: 63934228
Change-Id: Ia773afa93df00b32debd08f0a66893656f710341