Limit knownActivityEmbeddingCerts in manifest

Enforce manifest validation limits on knownActivityEmbeddingCerts to prevent
manifest bloat and potential performance degradation during signature
verification at app launch. Specifically, limit the number of certificates
to 50 and ensure each string is exactly 64 characters long (SHA-256 digest).

Bug: 429417238
Test: presubmit
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:5c47d86bbf21b44eced5551da433934109f10cd2
Merged-In: Ic24dff9c3ca6d1635c7b47ccdc1a8f003cc92830
Change-Id: Ic24dff9c3ca6d1635c7b47ccdc1a8f003cc92830