libcrypto_utils: switch to C++.

brillo is long gone, so no one should care about being C any more, and
this will let future janitorial work take advantage of RAII.

Test: treehugger
Change-Id: I06acd01e8b30247bed6e971ab3e8660d3e599cce
diff --git a/libcrypto_utils/Android.bp b/libcrypto_utils/Android.bp
index d7175e0..923b291 100644
--- a/libcrypto_utils/Android.bp
+++ b/libcrypto_utils/Android.bp
@@ -23,7 +23,7 @@
     },
     host_supported: true,
     srcs: [
-        "android_pubkey.c",
+        "android_pubkey.cpp",
     ],
     cflags: [
         "-Wall",