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/Android.bp b/libunwindstack/Android.bp
index e6d3bff..94f0f8e 100644
--- a/libunwindstack/Android.bp
+++ b/libunwindstack/Android.bp
@@ -44,6 +44,7 @@
 cc_library {
     name: "libunwindstack",
     defaults: ["libunwindstack_flags"],
+    export_include_dirs: ["include"],
 
     srcs: [
         "ArmExidx.cpp",