Use Apex sepolicy if it's available

[reland aosp/1910032]

Updatable sepolicy is shipped within an zip within an APEX. Because
it needs to be available before Apexes are mounted, apexd copies
the zip from the APEX and stores it in /metadata/sepolicy. If there is
not updatable sepolicy in /metadata/sepolicy, then updatable policy is
loaded from /system/etc/selinux/apex. Init performs the following
steps on boot:

1. Validates the zip by checking its signature against a public key
that is stored in /system/etc/selinux.
2. Extracts files from zip and stores them in /dev/selinux.
3. Checks if the apex_sepolicy.sha256 matches the sha256 of
precompiled_sepolicy. if so, the precompiled sepolicy is used.
Otherwise, an on-device compile of the policy is used. This is the
same flow as on-device compilation of policy for Treble.
4. Cleans up files in /dev/selinux which are no longer needed.
5. Restorecons the remaining files in /dev/selinux.
6. Sets selinux into enforcing mode and continues normal booting.

Bug: 199914227
Test: adb root
adb shell cp /apex/com.android.sepolicy.apex/etc/SEPolicy-33.zip \
/metadata/sepolicy/SEPolicy.zip
adb shell cp /apex/com.android.sepolicy.apex/etc/SEPolicy-33.zip.sig \
/metadata/sepolicy/SEPolicy.apk.sig
adb shell cp /apex/com.android.sepolicy.apex/etc/SEPolicy-33.zip.fsv_sig \
/metadata/sepolicy/SEPolicy.zip.fsv_sig
adb reboot
ls -laZ /dev/selinux/
Verify that files exist and that the labels are expected.

Change-Id: I2dbe43e329e91f46abedc7e44f7d24b220e19173