[FM] The radio text will disappear when stopping FM

Launch FM and play a station with radio text ->
Press stop button -> Check the radio text, radio text
disappear when stopping FM.

The fix is not to change radio text when stopping FM.
Do not set RDS DB empty when stopping RDS thread because
we will read it from DB.

https://partner-android-review.git.corp.google.com/#/c/188805/



Bug 18762207

Change-Id: Ib450806e96b28c02f1af02fcd084ede1ea5a056c
Signed-off-by: Benson Huang <benson.huang@mediatek.com>