Question: I have a log In page with username and password fields. I would like to know the best place to put the log-in button below them. Should it be left-aligned, centered, right-aligned or fill the entire width of the parent?

Is there a industry standard or best practice for Android Log In Buttons?

Answer: 
I would say put the login button below the fields. And center it, filling the same amount of space as the fields. This would look the best and would show up great in landscape and portrait.

A lot of companies put a lot of research into the best UI layouts.
The basic layout for login activity that I stumble upon everywhere in a books looks like this:

```
<TextView android:id="@+id/userNameLbl" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:text="Username: " 
    android:layout_alignParentTop="true" /> 

<EditText android:id="@+id/userNameText" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/userNameLbl" /> 

<TextView android:id="@+id/pwdLbl" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/userNameText" 
    android:text="Password: " /> 

<EditText android:id="@+id/pwdText" 
    android:layout_width="fill_parent"  android:layout_height="wrap_content" 
    android:layout_below="@id/pwdLbl" /> 

<Button android:id="@+id/btn" android:onClick="doClick"
       android:layout_width="fill_parent"  android:layout_height="wrap_content" 
       android:text="Login" /> 

```

So it's pretty much similar to what @Arkde tried to show you in the second link.

Hope this helps.