Address Mesh API feedback

Relnote: "Updated constants with appropriate
prefixes for TYPE and ALPHA_TYPE.
Updated IntDefs to be @hide w/ source retention
Updated javadoc formatting for Attibute
Added public accessors for Attribute and Varying
fields
Added toString methods for Attribute and Varying
Added size annotations for MeshSpecification.make APIs
Added docs to specify the default colorspace and alpha
type parameters used.
Changed List parameters for Arrays to avoid unnecessary
allocations"

Fixes: 266626021
Test: Re-ran mesh tests
Change-Id: I6954d654a12a99ac7ff6f560c9fd35274a904d06