Officially it's Ctrl+F11 & Ctrl+F12 or KEYPAD 7 & KEYPAD 9.
In practise it's a bit quirky.
Specifically it's Left Ctrl+F11 and Left Ctrl+F12 to switch to previous orientation and next orientation respectively.
You have to release Ctrl before you can rotate again.
KEYPAD 7 and KEYPAD 9 only work with Num Lock OFF (so they're acting as Home & PageUp rather than 7 & 9).
The only orientations are vertically upright and rotated one quarter-turn anti-clockwise.
Maybe a bit too much info for such a simple question, but it drove me half-mad finding this out.
Note: This was tested on Android SDK R16 and a very old keyboard, modern keyboards may behave differently.
