Bluetooth: follow system theme in dialogs

Bug: 188222741
Test: share a file via bluetooth with BT disabled, verify
´Turn on Bluetooth now´ and ´Turning on Bluetooth´ -dialogs
are dark
Change-Id: If24852e3731193089bdee4a2429fcd0cfbf385cf
Signed-off-by: Timi Rautamäki <timi.rautamaki@gmail.com>