SignApk: update SignApk to support SHA256withRSA signing

(cherry picked from commit c0581a02ede48007623030c4ce13174ee972cc42)

Change-Id: I6212e0f3a6aa3265b111f0f7348b667af4a668b0
Merge "SignApk: update SignApk to support SHA256withRSA signing"
Merge https://android.googlesource.com/platform/build