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/ElfTestUtils.cpp b/libunwindstack/tests/ElfTestUtils.cpp
index 81064dd..069386b 100644
--- a/libunwindstack/tests/ElfTestUtils.cpp
+++ b/libunwindstack/tests/ElfTestUtils.cpp
@@ -31,6 +31,8 @@
 
 #include "ElfTestUtils.h"
 
+namespace unwindstack {
+
 template <typename Ehdr>
 void TestInitEhdr(Ehdr* ehdr, uint32_t elf_class, uint32_t machine_type) {
   memset(ehdr, 0, sizeof(Ehdr));
@@ -136,3 +138,5 @@
                                                            TestCopyFuncType);
 template void TestInitGnuDebugdata<Elf64_Ehdr, Elf64_Shdr>(uint32_t, uint32_t, bool,
                                                            TestCopyFuncType);
+
+}  // namespace unwindstack