adb: static constexpr.

Also inline some single-use constants.

Bug: N/A
Test: ran tests
Change-Id: I1ac028667772599291da402120a270d0667fed04
diff --git a/adb/adb_utils.cpp b/adb/adb_utils.cpp
index ffac315..35017f0 100644
--- a/adb/adb_utils.cpp
+++ b/adb/adb_utils.cpp
@@ -49,9 +49,9 @@
 
 
 #if defined(_WIN32)
-constexpr char kNullFileName[] = "NUL";
+static constexpr char kNullFileName[] = "NUL";
 #else
-constexpr char kNullFileName[] = "/dev/null";
+static constexpr char kNullFileName[] = "/dev/null";
 #endif
 
 void close_stdin() {