Make the library usable as a library.

- Add namespace unwindstack everywhere so that it's easier for other
  code to use the library.
- Move some of the header files into include/unwindstack so that they
  can be exposed.
- Modify the headers so that only a limited number need to be exposed.
- Update the tools to use the new headers.
- Add a GetLoadBias() call on the Elf object. This prevents the need
  to get the interface object out of the Elf object.
- Move the GetRelPc() call out of the Reg class, to the Elf class. It's
  not always the case that a Reg object will be around when you want to
  get a relative pc. The tests for this moved to ElfTest.cpp.

Bug: 23762183

Test: Unit tests pass.
Change-Id: Iac609dac1dd90ed83d1a1e24ff2579c96c023bc3
diff --git a/libunwindstack/tests/DwarfSectionTest.cpp b/libunwindstack/tests/DwarfSectionTest.cpp
index 20f0e2a..fc67063 100644
--- a/libunwindstack/tests/DwarfSectionTest.cpp
+++ b/libunwindstack/tests/DwarfSectionTest.cpp
@@ -19,10 +19,12 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
-#include "DwarfSection.h"
+#include <unwindstack/DwarfSection.h>
 
 #include "MemoryFake.h"
 
+namespace unwindstack {
+
 class MockDwarfSection : public DwarfSection {
  public:
   MockDwarfSection(Memory* memory) : DwarfSection(memory) {}
@@ -158,3 +160,5 @@
 
   ASSERT_TRUE(mock_section.Step(0x1000, nullptr, &process));
 }
+
+}  // namespace unwindstack