Improve error messages related to signing.

- Print a human-readable error message when the keystore file does not
  eixst, instead of the default gradle @Input check.
- Migrate ValidateSigningTask to Java.
- Fail with a nice message if the key alias is not found in the
  keystore.

Bug: http://b.android.com/75989
Change-Id: I92e16a13beabcd416e22e36997cd6ef611e3d22b
3 files changed