Implement ReadAll(SharedFD, std::string*)

This allows reading the full contents of a SharedFD into a string. It
assumes that while the ReadAll function is running, the given SharedFD
will close.

Test: N/A
Bug: 137304531
Change-Id: I60935e7d5daed07cd484a2d207ebd34347fe9a52
diff --git a/common/libs/fs/Android.bp b/common/libs/fs/Android.bp
index 01e5f32..59075da 100644
--- a/common/libs/fs/Android.bp
+++ b/common/libs/fs/Android.bp
@@ -17,6 +17,7 @@
     name: "libcuttlefish_fs",
     srcs: [
         "gce_fs.cpp",
+        "shared_buf.cc",
         "shared_fd.cpp",
     ],
     shared: {
@@ -50,6 +51,7 @@
     ],
     srcs: [
         "gce_fs.cpp",
+        "shared_buf.cc",
         "shared_fd.cpp",
     ],
     shared_libs: [