[SYCL][NFC] Pass adapter by reference in backend[_impl]

[uditagarwal97]

It's a part of larger refactoring effort to pass adapter via reference instead of pointer everywhere in the codebase.

Follow-up of:
#19186
#19184

[maarquitos14]

LGTM, just one nit.

[uditagarwal97]

Failure in Windows BMG:

********************
Failed Tests (3):
  SYCL :: Adapters/level_zero/ext_intel_cslice.cpp
  SYCL :: Adapters/level_zero/ext_intel_queue_index.cpp
  SYCL :: Adapters/sycl-ls-uuid-subdevs.cpp

********************
Unexpectedly Passed Tests (5):
  SYCL :: bindless_images/array/fetch_handle_carray2d.cpp
  SYCL :: bindless_images/read_1D_O0.cpp
  SYCL :: bindless_images/sampled_fetch/fetch_1D_O0.cpp
  SYCL :: bindless_images/sampled_fetch/fetch_2D_O0.cpp
  SYCL :: bindless_images/sampling_1D_O0.cpp

Adapter tests were marked XFAIL in #19242 and Bindless images tests were fixed in #19215. All of these test failures are unrelated to this PR. Merging.