Revert "Disable SystemUiRavenTests for now"

This reverts commit 21f46b9e.

Reason for revert:

The test infra now runs individual ravenwood tests, so now
`SystemUiRavenTests` shouldn't be executed in pre or post-submit, because it's not in any TEST_MAPPINGs.

Previously, the test infra would always execute all ravenwood tests, when it was running any ravenwood test.

Fix: 356666754
Test: atest SystemUiRavenTests
Flag: EXEMPT host test change only
Change-Id: If5e0812b21dda3ae3064c7f88728a9b1aeaff9c0