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/ElfInterfaceArmTest.cpp b/libunwindstack/tests/ElfInterfaceArmTest.cpp
index 67c9a6b..c7ef4a1 100644
--- a/libunwindstack/tests/ElfInterfaceArmTest.cpp
+++ b/libunwindstack/tests/ElfInterfaceArmTest.cpp
@@ -20,12 +20,15 @@
 
 #include <vector>
 
+#include <unwindstack/Regs.h>
+
 #include "ElfInterfaceArm.h"
 #include "Machine.h"
-#include "Regs.h"
 
 #include "MemoryFake.h"
 
+namespace unwindstack {
+
 class ElfInterfaceArmTest : public ::testing::Test {
  protected:
   void SetUp() override {
@@ -370,3 +373,5 @@
   ASSERT_EQ(0x10U, regs.pc());
   ASSERT_EQ(0x10U, regs[ARM_REG_PC]);
 }
+
+}  // namespace unwindstack