Add pixel for process creation, for both main and VPN processes (#4208)

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Task/Issue URL:
https://app.asana.com/0/488551667048375/1206681496516579/f

### Description

Adds process creation pixels, for both main app and VPN process.

### Steps to test this PR

- [x] Add logcat filter `message~:"m_process_created"`

**Main app process**
- [x] Launch the app; verify you see `m_process_created_main`
- [x] Background and restore the app; verify you do **not** see it again
- [x] Kill the app and relaunch it; verify you **do** see it again

**VPN process**
- [x] onboard through AppTP
- [x] verify you see `m_process_created_vpn` in the logs

app/src/main/java/com/duckduckgo/app/pixels/AppPixelName.kt

@@ -20,6 +20,8 @@ import com.duckduckgo.app.statistics.pixels.Pixel
 
 enum class AppPixelName(override val pixelName: String) : Pixel.PixelName {
     APP_LAUNCH("ml"),
+    PROCESS_CREATED_MAIN("m_process_created_main"),
+    PROCESS_CREATED_VPN("m_process_created_vpn"),
 
     FORGET_ALL_PRESSED_BROWSING("mf_bp"),
     FORGET_ALL_PRESSED_TABSWITCHING("mf_tp"),

app/src/main/java/com/duckduckgo/app/pixels/ProcessCreationPixelSender.kt

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2024 DuckDuckGo
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.duckduckgo.app.pixels
+
+import androidx.lifecycle.LifecycleOwner
+import com.duckduckgo.app.lifecycle.MainProcessLifecycleObserver
+import com.duckduckgo.app.lifecycle.VpnProcessLifecycleObserver
+import com.duckduckgo.app.statistics.pixels.Pixel
+import com.duckduckgo.di.scopes.AppScope
+import com.squareup.anvil.annotations.ContributesMultibinding
+import dagger.SingleInstanceIn
+import javax.inject.Inject
+
+@ContributesMultibinding(
+    scope = AppScope::class,
+    boundType = MainProcessLifecycleObserver::class,
+)
+@SingleInstanceIn(AppScope::class)
+class ProcessCreationMainPixelSender @Inject constructor(
+    private val pixel: Pixel,
+) : MainProcessLifecycleObserver {
+
+    override fun onCreate(owner: LifecycleOwner) {
+        pixel.fire(AppPixelName.PROCESS_CREATED_MAIN)
+    }
+}
+
+@ContributesMultibinding(
+    scope = AppScope::class,
+    boundType = VpnProcessLifecycleObserver::class,
+)
+@SingleInstanceIn(AppScope::class)
+class ProcessCreationVpnPixelSender @Inject constructor(
+    private val pixel: Pixel,
+) : VpnProcessLifecycleObserver {
+
+    override fun onVpnProcessCreated() {
+        pixel.fire(AppPixelName.PROCESS_CREATED_VPN)
+    }
+}