init: Read previous state of securebits before modifying

When Android is running in a container, some of the securebits might be
locked, which makes prctl(PR_SET_SECUREBITS) fail.

This change gets the previous state of the process' securebits and adds
the desired bits.

Bug: 62388055
Test: aosp_bullhead-eng boots
Test: If init has non-zero securebits, it can also boot
Change-Id: Ie03bf2538f9dca40955bc58314d269246f5731bd