storageproxyd: Remove setuid, setgid, and capset

These operations require excessive SELinux and UNIX permissions.

Instead of dropping privileges after starting we will start
storageproxyd as "system" user.

Bug: 205904330
Test: com.android.storage-unittest.td
Change-Id: I0b2503a746c52474c8cc2e1f7a2fbe17c98d6d8b