Never call app_main more than once

Different kernels seem to handle ADDR_COMPAT_LAYOUT differently,
sometimes passing it to its children, sometimes not. If it's not
passed to its child successfully, we can end up in a restart loop.

Instead of testing for the presence of ADDR_COMPAT_LAYOUT, use an
environment variable instead, which is handled more predictably.

Bug: 8392487
Change-Id: Ia531dd2abb4e1cd46f3430d844e644f53581f530