[adb] Correctly read host/user name on Windows.

Bug: 162111908

Test: On linux/mac:
HOSTNAME="" adb keygen mykey; cat mykey.pub # Also LOGNAME
HOSTNAME="测试" adb keygen mykey; cat mykey.pub
HOSTNAME="test" adb keygen mykey; cat mykey.pub

Test: On Windows:
set COMPUTERNAME=&& adb keygen mykey
&& powershell -command "get-content -encoding utf8 mykey.pub" # Also USERNAME
set COMPUTERNAME=测试&& adb keygen mykey
&& powershell -command "get-content -encoding utf8 mykey.pub"
set COMPUTERNAME=test&& adb keygen mykey
&& powershell -command "get-content -encoding utf8 mykey.pub"

Change-Id: I62c952c511a620286aa4e05b1763361406e9f89a