Force-initialize potential padding area.

When collapsing our heap-and-stack boundaries, we round up to ensure
our padding math is still aligned on the remote side.  This rounding
can leak a small amount of uninitialized memory, so this change
force-initializes any potential padding area.

Bug: 213170822
Test: atest libandroidfw_tests
Change-Id: Ife6fcd1b74aaabd932a77ea42e0d0ad3a6b9cae6