Update OWNERS files for frameworks/base/boot

Adds OWNERS files for frameworks/base/boot and its new subdirectory
hiddenapi to include the Soong team and the compat team as they will be
the ones responsible for maintaining the contents of those directories.

A follow up change will move the frameworks/base/config/hiddenapi-*
into the new hiddenapi directory but that change will require separate
changes downstream. Separating this change out avoids having general
frameworks/base reviewers from having to review multiple different
changes for which they have little context.

Bug: 179354495
Test: m nothing
Change-Id: Ia57d69c5b6031ecab9fa3a27e293ce8166ef50d3