graphics.mapper@2.0: Document previously-implicit accessRegion behavior.

Existing Android framework code (and transitively, CTS test) require
that an accessRegion of (0,0,0,0) is treated the same as an
accessRegion covering the entire buffer, when calling lock() or
lockYCbCr().

Document this so that there is no confusion about this going forward,
since this requirement pre-dates the HIDL HALs.

Bug: 119440345
Test: Builds
Change-Id: Id16831d3da4ec3dc74dbdca18447581a50ee1193