Add a prefix for objc category.

According to the Google Objective-C style [1], category names should
start with an appropriate prefix. WebRTC has some category definitions
for system interfaces, but it doesn't use prefixes.

$ otool -ov WebRTC.framework/WebRTC | grep -E "^[0-9a-z]{16} 0x[0-9a-z]+ __OBJC_._CATEGORY" | grep -v "_RTC"
0000000002160840 0x217c3c0 __OBJC_$_CATEGORY_UIDevice_$_H264Profile
0000000002160850 0x21808b8 __OBJC_$_CATEGORY_AVCaptureSession_$_DevicePosition
0000000002160858 0x2180968 __OBJC_$_CATEGORY_NSString_$_StdString
0000000002160860 0x21809c8 __OBJC_$_CATEGORY_NSString_$_AbslStringView

To avoid conflicts, prefix the names and methods of those categories.
Also remove sdk/objc/Framework/Classes/Common/NSString+StdString.h as
it is not used by any other files.

[1] https://google.github.io/styleguide/objcguide.html#category-naming

Bug: webrtc:13884
Change-Id: I2cf2742af198ab4e0bfb15c0476d72971e50ceee
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262341
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36880}
diff --git a/sdk/objc/unittests/RTCIceServerTest.mm b/sdk/objc/unittests/RTCIceServerTest.mm
index 772653c..1b3a12c 100644
--- a/sdk/objc/unittests/RTCIceServerTest.mm
+++ b/sdk/objc/unittests/RTCIceServerTest.mm
@@ -17,7 +17,7 @@
 
 #import "api/peerconnection/RTCIceServer+Private.h"
 #import "api/peerconnection/RTCIceServer.h"
-#import "helpers/NSString+StdString.h"
+#import "helpers/NSString+RTCStdString.h"
 
 @interface RTCIceServerTest : XCTestCase
 @end
@@ -125,10 +125,10 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithNativeServer:nativeServer];
   EXPECT_EQ(1u, iceServer.urlStrings.count);
   EXPECT_EQ("stun:stun.example.net",
-      [NSString stdStringForString:iceServer.urlStrings.firstObject]);
-  EXPECT_EQ("username", [NSString stdStringForString:iceServer.username]);
-  EXPECT_EQ("password", [NSString stdStringForString:iceServer.credential]);
-  EXPECT_EQ("hostname", [NSString stdStringForString:iceServer.hostname]);
+            [NSString rtc_stdStringForString:iceServer.urlStrings.firstObject]);
+  EXPECT_EQ("username", [NSString rtc_stdStringForString:iceServer.username]);
+  EXPECT_EQ("password", [NSString rtc_stdStringForString:iceServer.credential]);
+  EXPECT_EQ("hostname", [NSString rtc_stdStringForString:iceServer.hostname]);
   EXPECT_EQ(2u, iceServer.tlsAlpnProtocols.count);
   EXPECT_EQ(2u, iceServer.tlsEllipticCurves.count);
 }