Use InstallRootKeyCertificate for keybox and cert

Merge from Widevine repo of http://go/wvgerrit/55461

This CL allows provisioning 3.0 devices to install their OEM certs
from an initialization partition.  This method is already used for
keyboxes on Android -- we are just adding the ability to use it for
OEM certs, also.

Also, for v15, we require OEMCrypto to report a valid certificate in
the unit tests.

bug: 111725154
test: unit tests
Change-Id: I142c84a1a67bdb4cee943cfd12a632421901eb24

libwvdrmengine/oemcrypto/test/oec_device_features.cpp

@@ -143,6 +143,7 @@ std::string DeviceFeatures::RestrictFilter(const std::string& initial_filter) {
   if (api_version < 12)               FilterOut(&filter, "*API12*");
   if (api_version < 13)               FilterOut(&filter, "*API13*");
   if (api_version < 14)               FilterOut(&filter, "*API14*");
+  if (api_version < 15)               FilterOut(&filter, "*API15*");
   // Some tests may require root access. If user is not root, filter these tests
   // out.
   if (getuid()) {