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

debuggerd.te

@@ -18,7 +18,7 @@ allow debuggerd {
   -keystore
   -ueventd
   -watchdogd
-}:process { ptrace getattr };
+}:process { execmem ptrace getattr };
 allow debuggerd tombstone_data_file:dir rw_dir_perms;
 allow debuggerd tombstone_data_file:file create_file_perms;
 allow debuggerd shared_relro_file:dir r_dir_perms;