Fix the texture ID reuse issue in HWUI.

Issue: When the layer of previous frame is destroyed, it doesn't clear the
texture id in mBoundTextures[mTextureUnit], so in the next frame, if
glGenTexture returns same texture ID of the previous frame,
the new texture is not bound.

CRs-fixed: 671736

Change-Id: Ifc5fd2115fb1863b3c006ab14b0faabeaeb4eab4