ANDROID: Fix color key handling for 16bit mouse cursors.

This should fix a issue similarly to bug #6108:
"WII: Zak FM-TOWNS mouse cursor encased in blue box".
ANDROID: Make transparent cursor pixels all 0 in 16bit cursor mode.

Since the Android backend uses glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA)
we need to make the transparent cursor pixels all 0 otherwise we might get
artifacts from blending.
Merge pull request #453 from lordhoto/android-16bit-cursor

ANDROID: Fix color key handling for 16bit mouse cursors.