Refactor String8 to std::string calls

DisplayViewport::uniqueId is now std::string, so change the calls
appropriately. Do some additional cleanups and conversions.
This almost completely removes the dependency on String8.

Test: build only
Bug: 111108021
Change-Id: Ibbb6ca59e9061954d4a5fb930ef03d42cb0230db
diff --git a/tools/validatekeymaps/Android.bp b/tools/validatekeymaps/Android.bp
index 6fb278c..819e75b 100644
--- a/tools/validatekeymaps/Android.bp
+++ b/tools/validatekeymaps/Android.bp
@@ -15,6 +15,7 @@
     ],
 
     static_libs: [
+        "libbase",
         "libinput",
         "libutils",
         "libcutils",