Use alpha channel instead of red in drawCachedGlyphBitmap

the correct value to copy in bitmap is alpha component from
the cachebuffer. currently it is offset to red channel if format is
RGBA which is wrong.

this is followup fix for https://android-review.googlesource.com/#/c/93943/

Change-Id: I96b5ba35df5ccaef44caf2542d35d6585ba1df80