A `SettingsSource` is meant to represent any kind of local or remote
store from which settings can be read, be this a local file, remote URL,
database, etc. It is concerned with reading in (and possibly decoding)
settings data, and computing a consistent hash key that may be used in
caching.
A `SettingsFormat` is meant to detect supported file types and/or decode
source contents into settings arrays. As of now, JSON is the only
supported format but others may be implemented.
`FileSource` is the first source implementation, with its default format
being JSON, meant to read settings from local JSON files.
`ArraySource` is mostly useful for testing using array literals.
Refactored `SettingsBuilder` methods to use the new source abstractions.
Bug: T295499
Change-Id: If7869609c4ad1ccd0894d5ba358f885007168972
