Most files on /system are labeled with the "system_file" label, and
are readable by default by all SELinux domains. However, select
executables are labeled with their own label, so that SELinux knows
what domains to enter upon running the executable.

Allow adbd read access to labeled executables in /system. We do
this by granting adbd read access to exec_type, the attribute
assigned to all executables on /system.

This allows "adb pull /system" to work without generating
SELinux denials.

Bug: 18078338
Change-Id: I97783759af083968890f15f7b1d8fff989e80604