Explicitly set proguard_compatibility for service-bluetooth

This currently defaults to proguard_compatibility, which is made explicit by this change.

This ensures that this target will not be affected by changing the default value of proguard_compatibility to false.

Bug: 215530220
Test: m + presubmit
Change-Id: I6a26cc592d18c08362bc0e5ad9787ebf07feb4ab