Separate the default label from the app provided

In Tile, we had a single label field. This could be set either:
* By the app, if they wanted a custom label
* By SystemUI, if there was no label, with the label from ServiceInfo.

This could cause a race condition, as we had no way of distinguishing
who had set it, so a change in locale would have the old default label
overwrite the new one.

With this change, separate these fields. However, we maintain the API
contract of `Tile.getLabel` returning what the UI will display (as it
was before this change).

Test: atest TileTest
Test: atest CtsTileServiceTestCases android.host.systemui
Test: atest CustomTileTest CustomTileStatePersisterTest
Fixes: 293811062
Change-Id: I700c1ac0160f637552e7106d43b6c8abeec41729