[android] ImageView - have height match width?

For people passing by now, in 2017, the new best way to achieve what you want is by using ConstraintLayout like this:

<ImageView
    android:layout_width="0dp"
    android:layout_height="0dp"
    android:scaleType="centerCrop"
    app:layout_constraintDimensionRatio="1:1" />

And don't forget to add constraints to all of the four directions as needed by your layout.

Build a Responsive UI with ConstraintLayout

Furthermore, by now, PercentRelativeLayout has been deprecated (see Android documentation).