Hide SettingsSlice provider

Explicitly set the Settings SliceBroadcastReceiver to
be non-exported and remove the intent-filter.

Add a second provider: SliceRelayReceiver to receive
broadcasts from SysUI to alert Settings to potential
changes to bound Settings Slices. The new receiver is
exported, but only notifies changes to Settings, and
doesn't make any changes itself.

Change-Id: I422c0b07a61efa8996e9fdfa398eee84bbc1796f
Merged-In: I80d070f7636614135ebe4f57a16f12a3eb6dee81
Fixes: 111330641
Test: boot, robolectric, Slicebrowser
(cherry picked from commit 6409cf5c)