Add a global elf cache.

Bug: 65682279

Test: Ran new unit tests.
Change-Id: I19c64614b2b11a27f58204d4cc34913c02e04c36
diff --git a/libunwindstack/tests/RegsTest.cpp b/libunwindstack/tests/RegsTest.cpp
index 15d5458..7c06373 100644
--- a/libunwindstack/tests/RegsTest.cpp
+++ b/libunwindstack/tests/RegsTest.cpp
@@ -184,7 +184,7 @@
   RegsMips64 regs_mips64;
   MapInfo map_info(0x1000, 0x2000);
   Elf* invalid_elf = new Elf(new MemoryFake);
-  map_info.elf = invalid_elf;
+  map_info.elf.reset(invalid_elf);
 
   regs_arm.set_pc(0x1500);
   EXPECT_EQ(0x500U, invalid_elf->GetRelPc(regs_arm.pc(), &map_info));