Introduce "adb keygen"

Introduce the "adb keygen" command.

  Usage: adb keygen <filename>

This command creates an adb public/private key pair in a user
specified file. This can be used to create new adb keys, or rotate
existing keys.

Modify adb's key generation routines to use the HOSTNAME/LOGNAME
environment variables if available. This allows someone to override
the username/hostname embedded within the adb public key file if
desired. Fallback to the old mechanisms if those environment
variables aren't available.

Bug: 18342715
Change-Id: Ibccee6088d4609aa05ad6687d3a1d8a8689d3e8a