Preserve explicit AndroidSdkHome when directory doesn't exist yet

[dalexsoto]

Problem

When calling AndroidSdkManager.Acquire() or SdkManager.DownloadSdk() with a target directory that doesn't exist yet (the typical fresh-install scenario), the operation fails with:

Android SDK Directory was not specified.

Root Cause

Both SdkTool and AndroidSdkManager constructors pass the user-specified path through SdkLocator.Locate(), which only returns paths where Directory.Exists() is true. When the target directory doesn't exist yet:

1. Locate() discards the user's path
2. It may return a different SDK found via env vars or known paths (e.g. ~/.android)
3. Or it returns nothing, leaving AndroidSdkHome as null
4. DownloadSdk() then throws because it has no target directory

Fix

When a path is explicitly provided via SdkToolOptions.AndroidSdkHome or the AndroidSdkManager(DirectoryInfo) constructor, honor it directly instead of passing it through Locate(). Auto-discovery via SdkLocator is only used when no path is specified.

Also removes a redundant re-assignment in the obsolete SdkTool(DirectoryInfo?) constructor that was overwriting the result from the delegated constructor.

Tests

Added SdkTool_PreserveHome_Tests with 6 tests covering:

• SdkManager preserves home when directory doesn't exist
• AndroidSdkManager preserves home when directory doesn't exist
• DownloadSdk doesn't throw "not specified" for non-existent directory
• Both types still work correctly with existing directories
• Auto-discovery still works when no path is specified