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/ArmExidx.h b/libunwindstack/ArmExidx.h
index 8c7f15a..f4361d4 100644
--- a/libunwindstack/ArmExidx.h
+++ b/libunwindstack/ArmExidx.h
@@ -21,6 +21,8 @@
 
 #include <deque>
 
+namespace unwindstack {
+
 // Forward declarations.
 class Memory;
 class RegsArm;
@@ -105,4 +107,6 @@
   bool pc_set_ = false;
 };
 
+}  // namespace unwindstack
+
 #endif  // _LIBUNWINDSTACK_ARM_EXIDX_H