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/ElfInterfaceTest.cpp b/libunwindstack/tests/ElfInterfaceTest.cpp
index 0f56ba8..acb7320 100644
--- a/libunwindstack/tests/ElfInterfaceTest.cpp
+++ b/libunwindstack/tests/ElfInterfaceTest.cpp
@@ -20,7 +20,9 @@
 
 #include <gtest/gtest.h>
 
-#include "ElfInterface.h"
+#include <unwindstack/ElfInterface.h>
+
+#include "DwarfEncoding.h"
 #include "ElfInterfaceArm.h"
 
 #include "MemoryFake.h"
@@ -33,6 +35,8 @@
 #define EM_AARCH64 183
 #endif
 
+namespace unwindstack {
+
 class ElfInterfaceTest : public ::testing::Test {
  protected:
   void SetUp() override {
@@ -921,3 +925,5 @@
 TEST_F(ElfInterfaceTest, init_section_headers_offsets64) {
   InitSectionHeadersOffsets<Elf64_Ehdr, Elf64_Shdr, ElfInterface64>();
 }
+
+}  // namespace unwindstack