Add KeyPurpose::ATTEST_KEY.

This allows applications to generate their own attestation keys and
then use them to attest other application-generated keys.

Bug: 171845652
Test: VtsAidlKeyMintTargetTest
Change-Id: I32add16dcc2d1b29665a88024610f7bef7e50200
diff --git a/security/keymint/aidl/vts/functional/Android.bp b/security/keymint/aidl/vts/functional/Android.bp
index 70f0b8a..7e78c45 100644
--- a/security/keymint/aidl/vts/functional/Android.bp
+++ b/security/keymint/aidl/vts/functional/Android.bp
@@ -21,6 +21,7 @@
         "use_libaidlvintf_gtest_helper_static",
     ],
     srcs: [
+        "AttestKeyTest.cpp",
         "KeyMintTest.cpp",
     ],
     shared_libs: [