adb: stop using adbkey.pub.

An adbkey/adbkey.pub pair that doesn't match up results in a
hard-to-diagnose scenario where "Always allow from this computer"
doesn't work. The private key contains all of the information that's
in the public key, so just extract the public key out of the private
key instead of storing them separately.

Bug: http://b/119634232
Test: rm ~/.android/adbkey.pub; adb kill-server; adb shell true
Test: rm ~/.android/adbkey*; adb kill-server; adb shell true
Change-Id: I0ae9033dbcd119c12cfb2b3977f1f1954ac800c1