Fixing test flakiness in Launcher initialization

Using same key for blob store and content provider based layouts

Using a single key for layout provider. This ensures that if one
test doesn't cleanup properly, any followup test which relies on
model initialization overrides the key appropriately

Bug: 370080120
Test: Presubmit
Flag: EXEMPT bugfix
Change-Id: Idef0a89a5d1ec89e24aae8a4549fd122634dad2f