fix: stop forcing size-only sync comparisons via RCLONE_LOCAL_NO_SET_MODTIME

[7MS8]

Split out of #406 per reviewer request (separate unrelated fixes into their own PRs).

Bug

RCLONE_LOCAL_NO_SET_MODTIME=true has been set for every rclone invocation since 2021 (f43949be), to work around chtimes crashes on old Android storage stacks (rclone#2446). It has a much bigger side effect than intended: rclone's local backend Precision() reports fs.ModTimeNotSupported unconditionally whenever NoSetModTime is set (backend/local/local.go), and rclone's default file comparison treats ModTimeNotSupported as "always equal" as soon as file sizes match (fs/operations/operations.go) — modification-time comparison is skipped entirely. Every sync, regardless of remote type or --checksum, silently degrades to a same-size-means-unchanged check. Changed files that happen to keep the same size are never re-synced.

Fix

Verified in current rclone (v1.71.0, vendored in this repo) that a failing chtimes()/SetModTime is not fatal: fs/operations/operations.go just logs and counts the error, the transfer itself still succeeds. The 2021 workaround for the old crash is no longer needed and was silently breaking change detection instead. Remove RCLONE_LOCAL_NO_SET_MODTIME=true from both Rclone.java and RcloneRcd.java, letting rclone's own runtime precision probe handle it per-remote.

Reproduction

• Any remote (tested against SFTP/Synology): sync a file once so source and destination match.
• Edit the file so its size stays exactly the same but content and mtime change (e.g. flip one character).
• Trigger sync again.
• Before this fix: sync.log shows the file is skipped ("Sizes identical" / unchanged) — the change is never picked up.
• After this fix: file is correctly detected as changed via modtime and re-transferred ("Copied (replaced existing)"); a subsequent sync then correctly no-ops.

Test plan

• Verified end-to-end against a live Synology SFTP remote and on a physical Pixel 7a (GrapheneOS): same-size/changed-content file is now correctly detected and re-transferred, no chtimes errors logged, local mtime ends up matching the source.
• Local build compiles (./gradlew :app:compileOssDebugSources).