Initialize all members of StagefrightPluginLoader

Function pointers should all be initialized to null.

Test: make cts -j123 && cts-tradefed run cts-dev -m \
CtsMediaTestCases --compatibility:module-arg \
CtsMediaTestCases:include-annotation:\
android.platform.test.annotations.RequiresDevice

Bug: 120785121
Change-Id: If9aa3c0ebd8888ea6e0b550d55397633657802fc