pdl: move ‘generate_field’ to ‘Field’

The ‘Field::from’ calls will eventually go away when we moved more
code to ‘Field’.

Test: atest pdl_tests pdl_inline_tests
(cherry picked from https://android-review.googlesource.com/q/commit:b79be8fa17c12d1aa814a387f2c33819a600979a)
Merged-In: I59c1d4c42fa47e512e037d7219ffa937a15c2be3
Change-Id: I59c1d4c42fa47e512e037d7219ffa937a15c2be3