Allow debuggerd execmem on debuggable domains

In anticipation of fixing a loophole in the Linux kernel that allows
circumventing the execmem permission by using the ptrace interface,
this patch grants execmem permission on debuggable domains to
debuggerd. This will be required for setting software break points
once the kernel has been fixed.

Bug: 31000401
Change-Id: I9b8d5853b643d24b94d36e2adbcb135dbaef8b1e