Define appl_trace_level needed for coverage instrumentation

Bug: 163819563

Define appl_trace_level even though LogMsg is trivial.  This is required
when coverage is enabled because the compiler is unable to eliminate the
`if` checks against appl_trace_level in APPL_TRACE_* macros.

Test: m CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS='system/bt' \
          net_test_hf_client_add_record
Change-Id: I0455d1f95146551f77ded662da1f5d85fb245c4e