Fix texture/sampler mapping in sRGB tests

Fix texture/sampler mapping in sRGB tests

sRGB decode tests assume that an API side texture with index i
is connected with a shader sampler variable uTexture with index i.

This relation was however incorrectly established, e.g.
the tests were mapping texture0 to a sampler with location 0, while
such sampler was not necessarily uTexture0.

This change fixes the mapping and makes the relation more obvious.

Components: AOSP
Affects:
dEQP-GLES31.functional.srgb_texture_decode.skip_decode.*.toggled
dEQP-GLES31.functional.srgb_texture_decode.skip_decode.*.conversion_gpu

Google bug: 38257749

Change-Id: I16508f6033b31583b1c5973b70a610dd59e78e8f