Disable GLES20Canvas on emu w/o native GL

When the emulator is run without '-gpu on', GLES20 isn't supported,
so claiming GLES20Canvas is available will lead to catastrophic
failure. This change makes GLES20Canvas available when compiled in
and either not running on the emulator, or running on the emulator
with native GL acceleration enabled.

Change-Id: I89c944f9e3c9585224f5aa0877335ea48ea4a468