Replace private bridge categories with private header

Summary:
public

A lot of the core modules have to use private methods in the bridge, specially
since the `RCTBatchedBridge` interface is never exposed. That was leading to a
lot of different private bridge categories spread across different modules,
which makes harder to identify which modules are affected by private API changes.

Replace all the categories with a single private header.

Reviewed By: nicklockwood

Differential Revision: D2757564

fb-gh-sync-id: 793158b9082d542b74a6094ed0db4d5dc3a88f78

Examples/UIExplorer/UIExplorerUnitTests/RCTAllocationTests.m

@@ -16,6 +16,7 @@
 #import <XCTest/XCTest.h>
 
 #import "RCTBridge.h"
+#import "RCTBridge+Private.h"
 #import "RCTContextExecutor.h"
 #import "RCTModuleMethod.h"
 #import "RCTRootView.h"
@@ -29,12 +30,6 @@ while ((CONDITION) && [timeout timeIntervalSinceNow] > 0) { \
 } \
 _Pragma("clang diagnostic pop")
 
-@interface RCTBridge (RCTAllocationTests)
-
-@property (nonatomic, weak) RCTBridge *batchedBridge;
-
-@end
-
 @interface RCTJavaScriptContext : NSObject
 
 @property (nonatomic, assign, readonly) JSGlobalContextRef ctx;