libmodprobe: Define InitModule helper

This commit introduces InitModule() helper function. The original
implementation of Modprobe::Insmod() called finit_module(2) internally,
but now it calls InitModule() instead. This change enables smoother
integration with the module dependency graph, which is introduced in the
next commit.

Bug: 421025789
Test: DUT boots
Change-Id: If5776b4343540b5b7a3d33ee80dc336a5460aebf