[DF-4794] Massive Reworking JNI calls!!!
Android uses native background threads for download manager update and notifications update.
JNIEnvironment should be taken each time it is needed! It is not stored as static anymore!
JNIEnv getting attaches thread to JVM if it is not attached!
Thread detaches from JVM when there is no objects which uses JNI calls.

JNINotification usage simplified.

ApplicationCore have background ticker (4 times per second when application is hidden).

Download Manager is not thread safe now. You should use it only in one thread!