SignApk - support loading private keys from pkcs#11 keystore

Summary:
Add two flags to load the keys from pkcs#11 keystore. When the option
-loadPrivateKeysFromKeyStore is specified, will load private keys from
the keystore with specified keyStoreName instead of load from file.

Test: make dist for arm_sunfish-user, which includes apk
and ota (wholefile) signing
Test:
- manually call signapk in Java11 (java9 may need additional
change to support), with statically registered pkcs#11 keystore, signed
both apk and ota-package.
- verified using apksigner and extracting otacert from ota-package, both
correct

Change-Id: I3efb8017f73d3d992c07ed4562acfef016a109fe