Fix rotation button background not showing after first boot.

KeyButtonView#setDarkIntensity() does not initialize mHasOvalBg and
mOvalBgPaint until the drawable is attached to the view, which happens
after KeyButtonView#draw() is called in the first run.

Test: Reboot, make sure background of the rotation button shows at first
run
Fixes: 134070810

Change-Id: Ib38660cde691f3d36f502023ffe082f0becd95eb