Add std::string system property functions.

Makes it easier to write correct code in a world where the maximum
property key/value lengths change.

Bug: http://b/23102347
Test: libbase_test64
Change-Id: I100f00904221bbcef9e8786a4e6e30428039bb49
diff --git a/base/Android.bp b/base/Android.bp
index e260412..88d8ad1 100644
--- a/base/Android.bp
+++ b/base/Android.bp
@@ -39,7 +39,10 @@
     shared_libs: ["liblog"],
     target: {
         android: {
-            srcs: ["errors_unix.cpp"],
+            srcs: [
+                "errors_unix.cpp",
+                "properties.cpp",
+            ],
             cppflags: ["-Wexit-time-destructors"],
         },
         darwin: {
@@ -78,6 +81,9 @@
         "test_main.cpp",
     ],
     target: {
+        android: {
+            srcs: ["properties_test.cpp"],
+        },
         windows: {
             srcs: ["utf8_test.cpp"],
             enabled: true,