sdkgen: evaluate 'emulator' path instead of 'android'

 set ANDROID_HOME by evaluating 'emulator' command instead of 'android'
 'android' is in path prebuilts/devtools/tools/android on source tree
 but 'emulator' is only specific to a PC installation of the sdk and is not
 present in the build tree. so we should check existence of sdk using 
 command -v emulator instead

Change-Id: Ib9b3584c4cd8b91a79d9c1f00f1f98ddb4b812a9
Signed-off-by: Arnav Gupta <championswimmer@aokp.co>