Set bin directories to 0751

Currently, /system/bin, /system/xbin, /product/bin, and /vendor/bin
are 0755, which allows any process to iterate through those
directories and list out the contents. For the vast majority of
processes, this is unnecessary. They only need to know whether a
particular binary exists or doesn't exist, but they don't need to
know the other binaries within those directories.

Allowing this is particularly problematic for SELinux. In particular,
some third party Android applications try to examine every file in
the bin directories, generating SELinux audit noise along the
way. This audit noise makes it harder to see real bugs, and falsely
implies an architectural dependency between the application and random
files in directories like /system/bin.

This change removes the ability to list the contents of the various bin
directories, preventing random probing by such apps. The ability to
execute files, or to probe a specific file by name, remain unchanged.

Addresses SELinux denials similar to the following:
  avc: denied { getattr } for comm="Thread-11" path="/system/bin/atrace" dev="dm-0" ino=189 scontext=u:r:untrusted_app_27:s0:c512,c768 tcontext=u:object_r:atrace_exec:s0 tclass=file permissive=0 app=uk.co.santander.santanderUK
  avc: denied { getattr } for comm="Binder:26637_2" path="/system/bin/atrace" dev="dm-0" ino=168 scontext=u:r:untrusted_app_25:s0:c512,c768 tcontext=u:object_r:atrace_exec:s0 tclass=file permissive=0 app=com.tencent.mm
  avc: denied { getattr } for comm="Thread-12" path="/system/bin/apexd" dev="dm-0" ino=451 scontext=u:r:untrusted_app_27:s0:c512,c768 tcontext=u:object_r:apexd_exec:s0 tclass=file permissive=1 app=com.grppl.android.shell.CMBlloydsTSB73

Shell access to these directories continues to be allowed, to allow for
host-side CTS tests.

Also adjust the indentation of some clang directives, to make the
presubmit hooks happy.

Test: Device boots and no apparent problems.
Change-Id: Ibe75682fac1983d39f3f479a5850ab5a96f6627d