Add install_symlink soong module type

This can be used to install symlinks to arbitrary locations/targets
on the device. Used to replace a make-built symlink.

Bug: 205632228
Test: built and ran the emulator observed the /system/bin/hwservicemanager symlink is still there
Change-Id: I6df922c8d919e6d56fa79702815a89c98f4d65ed