Refactor: remove ApexFileRepo::GetPublicKey()

It was to provide the pubkey of a preinstalled apex for verification.
But with block-apex and brand-new-apex support, it becomes more
complicated. Instead, when necessary, GetPreInstalledApex() can be used
to get the pubkey of the preinstalled apex.

Bug: 405014066
Bug: 179497746
Test: ApexTestCases
Change-Id: I0063a88c3b89c3f283ac0ee7d7c886eefd593424
5 files changed