Clarify error messages in Android Keystore Secure Area. (#399)

The current text assumes the failure has to do with key purpose, but
this is not the case and we're actively running into this error message
in the context of brokenness in Android Keystore wrt. Ed25519 and
X25519 support. With this fix, it's clear what the error is.

Test: Manually test.

identity-android/src/main/java/com/android/identity/android/securearea/AndroidKeystoreSecureArea.java

@@ -444,7 +444,7 @@ static String getSignatureAlgorithmName(@Algorithm int signatureAlgorithm) {
             }
             throw new IllegalStateException(e.getMessage(), e);
         } catch (InvalidKeyException e) {
-            throw new IllegalArgumentException("Key does not have purpose KEY_PURPOSE_SIGN", e);
+            throw new IllegalArgumentException(e);
         }
     }
 
@@ -484,7 +484,7 @@ static String getSignatureAlgorithmName(@Algorithm int signatureAlgorithm) {
             }
             throw new IllegalStateException(e.getMessage(), e);
         } catch (InvalidKeyException e) {
-            throw new IllegalArgumentException("Key does not have purpose KEY_PURPOSE_AGREE_KEY", e);
+            throw new IllegalArgumentException(e);
         }
     }