Activate encryption for /data/app-staging folder

Without enabling the encryption on this folder, we will not be able to
rename files from this folder to /data/app folder, since /data/app
folder is encrypted. Trying to rename files between unencrypted folder
to encrypted folder throws EXDEV error.

Turning on encryption for /data/app-staging has the following concerns:

1. Turning on the encryption will erase all of its content. But this is fine
since during OTA we fail all staged sessions anyway.
2. We need to create hardlinks from /data/app-staging to
/data/apex/active. This is also fine since we will be creating link from
encrypted folder (/data/app-staging) to non-encrypted folder and this
does not throw EXDEV error.

Bug: 163037460
Test: atest StagedInstallTest
Change-Id: Ie78f6df0c0e08de54a39c5e406957ad0a56b7727