Add feature flag to gate purpose declaration enforcement.

This flag is intended to be used for gating anything that relates to
purpose declration enforcement include parsing changes, policy changes,
and any public APIs (if needed).

Separate flags will be created later for enrolling various APIs into PD.

Bug: 417788696
Test: m -j99
Flag: EXEMPT adding flag
Change-Id: I1ea9e7512064246ca036c69d8378c57e2f05175a