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:1efb05a40b19498f1815af687dcd177b9c961df4
Merged-In: I50fbaf456f7f1b359b6f86cae3daa6ff2bbbf9c6
Change-Id: I50fbaf456f7f1b359b6f86cae3daa6ff2bbbf9c6