Add trackConversion for sending conversion events

[randyriback]

Summary

• Adds ParselyTracker.trackConversion(url, conversionType, conversionLabel, ...) for firing conversion events (newsletter signups, subscriptions, purchases, link clicks, lead capture, custom). Internally constructs an event with action="conversion" and merges _conversion_type and _conversion_label into extra_data, which the Parse.ly conversions backend (conversions-realtime) extracts and uses as the goal identity.
• New ConversionType enum mirrors the categories accepted server-side (newsletter_signup, lead_capture, link_click, subscription, purchase, custom).
• Reuses the existing EventsBuilder / InMemoryBuffer / FlushQueue / ParselyAPIConnection pipeline — zero changes to the network layer, the buffer, or the flush manager.
• Example app gains two sandbox buttons used during end-to-end verification.

This is the Android counterpart to the iOS SDK's trackConversion (parallel PR on AnalyticsSDK-iOS).

Test plan

Unit tests (all passing)

• EventsBuilderTest.when building conversion event, then build the correct one — verifies the built event has action == "conversion", the right url and idsite, no pvid / vsid, and that _conversion_type / _conversion_label plus caller-supplied keys land in data.
• ParselyTrackerTest.given tracker initialized, when calling trackConversion, do not throw any exception — smoke test for the public entry point (mirrors the existing call-doesn't-throw tests for other tracking methods).

Full suite (./gradlew :parsely:testDebugUnitTest): EventsBuilderTest 11 tests / ParselyTrackerTest 5 tests / 0 failures.

Manual end-to-end (Android emulator: Pixel 5, android-34, default arm64-v8a)

• Built and installed the example app on the emulator.
• Flipped dryRun to false temporarily in MainActivity.java to exercise the real network path.
• Tapped Track Pageview (sandbox) and Track Conversion (sandbox) against the sandbox.joshhanson.io apikey.
• Verified the on-wire JSON via adb logcat Parsely:V '*:S':

  {
    "action": "conversion",
    "url": "https://sandbox.joshhanson.io/path/test-conversion2",
    "idsite": "sandbox.joshhanson.io",
    "data": {
      "_conversion_type": "subscription",
      "_conversion_label": "android_sdk_smoke_test",
      "plan": "weekly",
      "source": "android_demo_app",
      "os": "android",
      "os_version": "34",
      "parsely_site_uuid": "...",
      "ts": 1778782087996
    }
  }

Testing Screenshots

[codecov]

Codecov Report

❌ Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.46%. Comparing base (6c22480) to head (0025deb).

Files with missing lines	Patch %	Lines
...m/parsely/parselyandroid/ParselyTrackerInternal.kt	78.57%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
+ Coverage   71.32%   73.46%   +2.14%     
==========================================
  Files          22       23       +1     
  Lines         415      441      +26     
  Branches       50       52       +2     
==========================================
+ Hits          296      324      +28     
+ Misses        105       99       -6     
- Partials       14       18       +4     

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