Fix potential bad info in eh_frame_hdr.

Due to a bug, an elf can have FDEs with a length of zero, while still
having another FDE for the same pc with a non-zero length. The
eh_frame_hdr can sometimes point to the zero length FDE, but it should
have pointed to the non-zero length FDE. In order to fix this, if the
eh_frame_hdr points at the zero length FDE then try and find the real FDE
directly from eh_frame.

The change cleans up and removes unused variables from DwarfEhFrameWithHdr
and changes the objects so that all of the DwarfSection objects and
DwarfEhFrameWithHdr object inherit from the same class.

Add new unit tests to verify this functionality.

Bug: 142483624

Test: Unit tests all pass.
Change-Id: I128a916e3ba378931de7d44ee15e57e24d4073df
diff --git a/libunwindstack/DwarfEhFrame.h b/libunwindstack/DwarfEhFrame.h
index df441fb..7a41e45 100644
--- a/libunwindstack/DwarfEhFrame.h
+++ b/libunwindstack/DwarfEhFrame.h
@@ -25,9 +25,9 @@
 namespace unwindstack {
 
 template <typename AddressType>
-class DwarfEhFrame : public DwarfSectionImplNoHdr<AddressType> {
+class DwarfEhFrame : public DwarfSectionImpl<AddressType> {
  public:
-  DwarfEhFrame(Memory* memory) : DwarfSectionImplNoHdr<AddressType>(memory) {}
+  DwarfEhFrame(Memory* memory) : DwarfSectionImpl<AddressType>(memory) {}
   virtual ~DwarfEhFrame() = default;
 
   uint64_t GetCieOffsetFromFde32(uint32_t pointer) override {