Make TogglableFlag backed by DeviceConfig for e2e testing

Bug: 138964490

TL;DR;; need this to be part of QQ1 or QD1 to verify if DeviceConfig
can be supported for launcher toggleableFlags.

Not handled in this CL:
- When flag is locally modified, that will override the flag value
How that scenario is handled should be discussed separately and is not
within scope of this CL.

Change-Id: I2e6694a40bee9202ed0b0d559e3b5607634071bf