Improve documentation about the user's default locale.

Add a bit of text to Locale's class documentation, and include a link from
every method that uses Locale.getDefault(). Also try to consistently say
"user's default locale", as a subliminal hint that this isn't necessarily
the developer's locale, nor en_US.

Bug: 2593000
Change-Id: Ieebe864ed6b9fecbfef5d5415269299739cedd1b
16 files changed