Revert^2 "[SettingsProvider] @Readable annotation to restrict access to hidden keys"

New in this revert:
+ Added missing annotations.
+ Added check for system apps.

BUG: 175024829
Test: atest android.appsecurity.cts.ReadableSettingsFieldsTest
Test: atest android.app.cts.NotificationTemplateTest
Test: atest com.android.systemui.statusbar.notification.row.ExpandableNotificationRowTest

1ca518c1ec75b8307126991ae2d253911b2be10c
Change-Id: I3f270831692b5ba0841e85b84902d0b101ff1095

Change-Id: I4cab132c1ef10f4319b570e8e9e15dcde6d909f8