Fix Javadoc of DevicePolicyManager#setStorageEncryption to reflect that

it's a no-op (and silent fail) when called from a non-system user.

DevicePolicyManagerService#setStorageEncryption also has its Javadoc
fixed.

Bug: 74591983
Change-Id: Ie191ad255cbfcade921d172641f134d6c49a17b1
Fixes: 74591983
Test: N/A - just javadoc changed