unwindstack: add some perfunctory MemoryOffline tests.

Add some tests for the behavior of MemoryOffline.

Test: libunwindstack_test 32/64 on hikey960
Change-Id: Ib65a0fe5347b86062ffeaf0adaf9af57066edb76
diff --git a/libunwindstack/Android.bp b/libunwindstack/Android.bp
index 21dd306..01804a8 100644
--- a/libunwindstack/Android.bp
+++ b/libunwindstack/Android.bp
@@ -136,6 +136,7 @@
         "tests/MemoryFake.cpp",
         "tests/MemoryFileTest.cpp",
         "tests/MemoryLocalTest.cpp",
+        "tests/MemoryOfflineTest.cpp",
         "tests/MemoryRangeTest.cpp",
         "tests/MemoryRemoteTest.cpp",
         "tests/MemoryTest.cpp",