Camera: Extend FEATURE_CAMERA_CONCURRENT to old API levels

Since running the front and back camera at the same time has been
possible since forever, there's no reason devices on older API levels
can't declare the FEATURE_CAMERA_CONCURRENT flag, even with the new
query APIs not present.  Explicitly document that the flag can be set
on API level 29 or earlier, and what it means.

Test: m offline-sdk-docs
Bug: 77960042
Change-Id: I186cb53d95debcc62c98afdef8c629bd9c6a5919