odsign: Don't initialize if we can't find a Keymaster.

This code will soon be obsolete, but make it resilient against a valid
keymaster instance not being found.

Bug: 180861644
Test: N/A
Change-Id: Ibf7e5259d08e4b1224fa13406ca4cdf641061946
diff --git a/ondevice-signing/Keymaster.cpp b/ondevice-signing/Keymaster.cpp
index d43828a..267dec8 100644
--- a/ondevice-signing/Keymaster.cpp
+++ b/ondevice-signing/Keymaster.cpp
@@ -68,7 +68,7 @@
 
     mDevice = devToUse;
 
-    return true;
+    return mDevice != nullptr;
 }
 
 std::optional<Keymaster> Keymaster::getInstance() {