Fix GPX file picker on Android 13+ and transport selection bug

[betodealmeida]

Summary

• Fix GPX file selection on Android 13+ by using ACTION_OPEN_DOCUMENT without restrictive MIME type filters
• Add file extension validation to ensure only .gpx files are accepted
• Fix transport spinner bug that was causing errors when selecting a transport mode

Problem

On Android 13+, GPX files were visible in the file picker but could not be selected. This was because:

1. The previous implementation used Intent.createChooser() which interferes with the Storage Access Framework
2. GPX files may have various MIME types (application/gpx+xml, application/octet-stream, text/xml, etc.) that didn't match the filter

Additionally, selecting a transport mode (Boat, Walk, etc.) caused an error due to a typo using read.getSelectedItemPosition() instead of transport.getSelectedItemPosition().

Changes

• Use ACTION_OPEN_DOCUMENT with CATEGORY_OPENABLE and proper permission flags
• Remove MIME type restrictions (GPX files have inconsistent MIME types across devices)
• Add isGpxFile() method to validate file extension after selection
• Add trip_invalid_file_type string resource for user-friendly error message
• Fix transport spinner variable name typo

Test plan

• Create a Trip post on Android 13+ device
• Tap "Load GPX" and verify GPX files are selectable
• Verify non-GPX files show "Please select a GPX file (.gpx)" error
• Select a transport mode and verify the Trip posts successfully