Metrics: Move bluetooth.proto to system/bt/proto

* Move bluetooth.proto to system/bt/proto/ and compile it for both
  Java LITE runtime and C++ LITE runtime so that it can be shared
  between Java and native code
* Modify Bluetooth code to use builder for protobuf messages
* Modify LE scan logging to log number of scan results at the end of a
  scan session
* Modify AdapterService to merge native metrics before Java metrics so
  that Java metrics can overwrite native metrics if needed

Bug: 33693818
Test: make, toggle Bluetooth, clearcut server-client E2E test
      adb shell dumpsys bluetooth_manager --proto-bin
      ACTS tests: BtMetricsTest, BtFunhausMetricsTest
Change-Id: I028df93d64faa58e7be6603a5a436517b9ebe297