binder: X509 are serialized to vector<uint8_t>

Change it from a string to a vector<uint8_t>. For DER
format, it is not a print-able string.

Test: pass
Bug: 198833574
Change-Id: I6b4d0ebc2d7429e927f7ca5abccc3f6aedfe1e6a
7 files changed