installd: enable SELinux restrictions

This change enables SELinux security enforcement on the installd
process.

For the installd.te file only, this change is equivalent to reverting
the following commits:

    * 50e37b93
    * 77d4731e

No other changes were required.

Testing: As much as possible, I've tested that package installation
works, from both adb and via Android market. There were no denials
in the kernel dmesg log, and everything appears to be working correctly.

It's quite possible I've missed something. If we experience problems,
I'm happy to roll back this change.

Bug: 9662644
Change-Id: Id93d4ee7b517dfa28c9a0b1d45d936b56892ac0a