Use GL_ALPHA for GLES2 compat in ByteBufferTest

Replace usage of GL_R8 and GL_RED with GLES2 core GL_ALPHA
in order to remove implicit dependency on GL_EXT_texture_rg.

Bug: 197667183
Change-Id: I99fb57ad144b2dd0dea82b65ce7cf147645856ca
Test: atest CtsOpenGLTestCases
1 file changed