adb: add adbd -> shell signal permissions.

adbd needs to kill spawned subprocesses if the client terminates
the connection. SIGHUP will be used for this purpose, which
requires the process:signal permission.

Bug: http://b/23825725
Change-Id: I36d19e14809350dd6791a8a44f01b2169effbfd4

adbd.te

@@ -9,8 +9,9 @@ userdebug_or_eng(`
 
 domain_auto_trans(adbd, shell_exec, shell)
 
-# Do not sanitize the environment or open fds of the shell.
-allow adbd shell:process noatsecure;
+# Do not sanitize the environment or open fds of the shell. Allow signaling
+# created processes.
+allow adbd shell:process { noatsecure signal };
 
 # Set UID and GID to shell.  Set supplementary groups.
 allow adbd self:capability { setuid setgid };