[FIX] Upload buttons on Android (#541)

android/app/src/main/res/drawable-hdpi/mention_header.png

android/app/src/main/res/drawable-mdpi/mention_header.png

android/app/src/main/res/drawable-xhdpi/mention_header.png

android/app/src/main/res/drawable-xxhdpi/mention_header.png

android/app/src/main/res/drawable-xxxhdpi/mention_header.png

app/containers/Button/index.js

app/containers/MessageBox/UploadModal.js

app/containers/MessageBox/index.js

app/i18n/locales/en.js

app/i18n/locales/pt-BR.js

app/views/RoomView/Header/index.js

...ages.xcassets/Icons/mention_header.imageset/Contents.json

...xcassets/Icons/mention_header.imageset/mention_header.png

...ssets/Icons/mention_header.imageset/mention_header@2x.png

...ssets/Icons/mention_header.imageset/mention_header@3x.png