[Pod] Move pod source files to pods/src/ instead of just pods/.

Unit tests work much better in Gradle if they're in src/, and the
production code works equivalent in Gradle and Soong if they're in just
pods/ or in pods/src. So, let's use src/ in both places to keep it
unified.

Bug: 307607958
Flag: EXEMPT refactor
Test: m SystemUI-application; mp sysuig
Change-Id: I4913b4ceb4710d3c86cccd469f11ed71f7289efe