fix(clock): Avoid generating timers with IDs that conflict with native

This commit attempts to ensure that the timers created by jasmine mock
clock do not conflict with the native timers. This also retains
pre-existing behavior whereby a native scheduled function cannot be
cleared if it was created prior to the mock clock being installed
(unless the mock clock is uninstalled first).

Prior to this commit, attempting to clear a native timer would result in
clearing a mocked scheduled function instead, in some scenarios where
the IDs conflicted.

fixes #2068

src/core/DelayedFunctionScheduler.js

@@ -6,7 +6,7 @@ getJasmineRequireObj().DelayedFunctionScheduler = function(j$) {
     this.scheduledLookup_ = [];
     this.scheduledFunctions_ = {};
     this.currentTime_ = 0;
-    this.delayedFnCount_ = 0;
+    this.delayedFnStartCount_ = 1e12; // arbitrarily large number to avoid collisions with native timer IDs;
     this.deletedKeys_ = [];
 
     this.tick = function(millis, tickDate) {
@@ -38,7 +38,7 @@ getJasmineRequireObj().DelayedFunctionScheduler = function(j$) {
       }
 
       millis = millis || 0;
-      timeoutKey = timeoutKey || ++this.delayedFnCount_;
+      timeoutKey = timeoutKey || ++this.delayedFnStartCount_;
       runAtMillis = runAtMillis || this.currentTime_ + millis;
 
       const funcToSchedule = {