Add prng_seeder to base PRODUCT_PACKAGES.

Needs to be on all (for now) devices with libcrypto, i.e. all
of them.

I failed to learn from https://r.android.com/1305853 that
adding something to Conscrypt's requirements doesn't work if
the module is coming from a prebuilt...

Again, the correct solution here would actually be to make
libcrypto rather than Conscrypt require prng_seeder but Soong
considers that a circular dependency

Bug: 243933553
Test: m installclean && m and verify prng_seeder is installed and running.
Change-Id: Ie34f8c693469eaeedae6a010b956d6c6513a6b15