Fix SOLID_COLOR in CPU composer

This was previously using SetPlane() which seems to just be based
on using a uint8_t value. I'm not sure why I used this in
aosp/2482638 originally...

Bug: b/400475306
Test: atest CuttlefishHwcTests
Change-Id: I2bd9fa910bdcf75d97b5422d91829218d2a07ad6
1 file changed