Replace Bluetooth reflection usages with impl @hide API.

Since we are targeting Android T and above, the reflection limitation
of AdvertisingSet#getOwnAddress() only works when targeting Android O
below. This change use "framework-bluetooth.impl" to access @hide APIs
in "framework-bluetooth" just like SL4A does:
https://cs.android.com/android/platform/superproject/+/master:external/sl4a/Common/Android.bp

This change also replace some constants with @SystemAPI usages.

Test: Maunal launch NearbyFastPairProviderSimulatorApp and pair with mainline FP seeker
Test: http://recall/-/cycSROwIosTzHgYDSIqPXl/fAWLYYQQMVCw0QWuefBU7v
Bug: 214015364
Ignore-AOSP-First: nearby_not_in_aosp_yet
Change-Id: Iedd22c05803e93da529936f03f911c0e647e84ec
diff --git a/nearby/tests/multidevices/clients/proguard.flags b/nearby/tests/multidevices/clients/proguard.flags
index fd494a8..11938cd 100644
--- a/nearby/tests/multidevices/clients/proguard.flags
+++ b/nearby/tests/multidevices/clients/proguard.flags
@@ -3,8 +3,8 @@
      *;
 }
 
-# Keep simulator reflection callback.
--keep class android.nearby.fastpair.provider.** {
+# Keep AdvertisingSetCallback#onOwnAddressRead callback.
+-keep class * extends android.bluetooth.le.AdvertisingSetCallback {
      *;
 }