Update Android's UploadFiles boundary to use random UUID

[nikoWah]

Changes made in UploadFiles for android:

• WebGateway security setting blocked file uploads from Android as it found the fixed asterisks boundary string to be duplicated requests.
• Update boundary to use random UUID instead of using fixed asterisks to be consistent with iOS implementation.

[bachbonglong]

Pls merge this code. I need boundary not *****

[tero-paananen]

Chunked file upload may stuck if this PR is missing. I will investigate more and i will take this PR in use...