Metrics: Dump native metrics to Java as std:string

* Dump metrics to Java layer as std::string instead of going through a
  file descriptor so that Java layer can process metrics data directly
* Add new method dumpMetrics(std::string*)

Bug: 33693818
Test: adb shell dumpsys bluetooth_manager --proto-bin
      SL4A metrics tests: BtMetricsTest, BtFunhausMetricsTest
Change-Id: I67883c1371862d7e4ff33cec01d710d2d8f274c9