disable module loading after BOOT_COMPLETED broadcast

This CL disables module loading by writing 1 to
/proc/sys/kernel/modules_disabled when the property sys.boot_completed
is set to 1 by ActivityManagerService (at the broadcast of
PHASE_BOOT_COMPLETED).

Bug: 36515654
Test: tested on sailfish and verified that module loading is disabled in
userdebug and enabled in eng mode
Merged-In: Id38d34a6395966ab21e440614337c0cfca791ad0
(cherry picked from commit 6ed19d16)

Change-Id: I2faa459b450b3a64f854c832c6b91cbf682bbffa