More robust jarsigner locator, removes need to hard-code build-tools versions in plugin

Improved zipalign locator to be future-proof against new build-tools versions. No longer requires a hard-coded list of versions.

Note: This PR is from a repository that was forked from https://github.com/testfairy/testfairy-gradle-plugin/issues/21 (also see https://github.com/testfairy/testfairy-gradle-plugin/pull/25).