Fail fast and document downloads on Android 9 and older

[codenameakshay]

On Android 9 and older, inserting into the shared Pictures collection needs WRITE_EXTERNAL_STORAGE. The plugin deliberately does not declare it, so downloadWallpaper downloaded the whole image and then failed with SecurityException: Permission Denial.

 downloadToMediaStore(url)
+  if SDK < 29 and WRITE_EXTERNAL_STORAGE is not granted
+    log the reason; return false            (nothing downloaded)
   download → validate → MediaStore insert

The README and the Android guide now say what an app that supports those versions must declare (maxSdkVersion="28") and request at runtime. The plugin's own manifest is unchanged.

Verification

On an API 26 emulator:

• Without the permission: the call fails before downloading, and the log explains why. No MediaStore row is created.
• With the permission (the host declares and grants it, using a temporary example patch that was not committed): the download succeeds and a MediaStore row is created.

Local CI (format, analyze, tests, pigeon check, Gradle test/lint/assemble, iOS simulator build): pass.

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