Added: Add ShellCommandShellEnvironment and TermuxShellCommandShellEnvironment to export ExecutionCommand variables

This adds onto f102ea20 to build termux environment. Variables for `ExecutionCommand` app have the `SHELL_CMD__` scope. Docs will be provided for details of the variables.

- `SHELL_CMD__SHELL_ID`
- `SHELL_CMD__SHELL_NAME`
- `SHELL_CMD__APP_SHELL_NUMBER_SINCE_BOOT`
- `SHELL_CMD__TERMINAL_SESSION_NUMBER_SINCE_BOOT`
- `SHELL_CMD__APP_SHELL_NUMBER_SINCE_APP_START`
- `SHELL_CMD__TERMINAL_SESSION_NUMBER_SINCE_APP_START`

The commit also adds `SystemEventReceiver` to Termux app that will receive `ACTION_BOOT_COMPLETED`.

app/src/main/AndroidManifest.xml

@@ -34,6 +34,7 @@
     <uses-permission android:name="android.permission.DUMP" />
     <uses-permission android:name="android.permission.WRITE_SECURE_SETTINGS" />
     <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
+    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
     <uses-permission android:name="android.permission.PACKAGE_USAGE_STATS" tools:ignore="ProtectedPermissions" />
 
     <application
@@ -169,6 +170,14 @@
             android:name=".app.TermuxOpenReceiver"
             android:exported="false" />
 
+        <receiver
+            android:name=".app.event.SystemEventReceiver"
+            android:exported="false">
+            <intent-filter>
+                <action android:name="android.intent.action.BOOT_COMPLETED" />
+            </intent-filter>
+        </receiver>
+
         <receiver
             android:name=".shared.activities.ReportActivity$ReportActivityBroadcastReceiver"
             android:exported="false" />