Fixed crash in file transfer due to backgroundtaskid not being initialized + removed unused header in jni

[Abstract API] Blacklists linphone_factory_create_core_2() and linphone_factory_cerate_core_with_config_2() while parsing

This must be done since these functions must be manually wrapped for each targeted language.

add helper class in java to access specific functions available only in Android SDK, such as retrieving DNS servers.
Refactor wifi lock, multicast lock so that they use this new class.
Manage the CPU lock required for call in the core, instead of app's LinphoneManager.

content_set_buffer now takes a uint8_t* instead of a void* so it can be correctly wrapped automatically

Unblacklisted 2 methods for Java wrapper because if getter is blacklisted, setter won't be wrapped...