Improve OWNERS coverage across frameworks/base/.

As general background, OWNERS files expedite code reviews by helping
code authors quickly find relevant reviewers, and they also ensure
that stakeholders are involved in code changes in their areas.

Some teams under frameworks/base/ have been using OWNERS files
successfully for many years, and we're ready to expand them to cover
more areas.  Here's the historical coverage statistics for the last
two years of changes before these new OWNERS changes land:

-- 56% of changes are fully covered by OWNERS
-- 17% of changes are partially covered by OWNERS
-- 25% of changes have no OWNERS coverage

Working closely with team leads, we've now identified clear OWNERS on
a per-package basis, and we're using "include" directives whenever
possible to to simplify future maintenance.  With this extensive
effort, we've now improved our coverage as follows:

-- 98% of changes are fully covered by OWNERS
-- 1% of changes are partially covered by OWNERS
-- 1% of changes have no OWNERS coverage

This specific change begins defining top-level OWNERS lists,
including a general catch-all for string translations.

Bug: 174932174
Test: manual
Exempt-From-Owner-Approval: refactoring with team leads buy-in
Change-Id: Ie7ac3302d40a717fa048115cca2ea4359de64959