Enable Flow Strict Local for 13 files in xplat/sonar

Summary:
First add a Flow Strict config, then enable Flow Strict Local for 13 files in xplat/sonar.

each change can land seperately:
drop-conflicts

Reviewed By: passy

Differential Revision: D13368791

fbshipit-source-id: 5bc1f9d3a179f4b9a554cc8cc629db0fda468a36

.flowconfig

@@ -22,5 +22,14 @@ module.name_mapper='flipper' -> '<PROJECT_ROOT>/src/index.js'
 
 suppress_type=$FlowFixMe
 
+[strict]
+deprecated-type
+nonstrict-import
+sketchy-null
+unclear-type
+unsafe-getters-setters
+untyped-import
+untyped-type-import
+
 [version]
 ^0.86.0