adb: add support for vendor key directories.

Allow directories to be specified in ADB_VENDOR_KEYS. On Linux, monitor
this directory for new keys to be added.

Additionally, deduplicate keys by hashing their public key.

Bug: http://b/29273531
Bug: http://b/30927527
Change-Id: I8d3312b216b7f2c11900f2235f1f1b1d1c7aa767
Test: manually tested by adding a key to a directory, and verifying
      that devices became authorized after replugging.