[settings] Remove @Readable for hidden fields added in S

This protects newly added fields from being accessed outside
system_server or system/privileged apps.

BUG: 175024829
Test: atest android.appsecurity.cts.ReadableSettingsFieldsTest
Change-Id: I40fda59c164a5982e811d51a69dc9aa8365b04d1
1 file changed