Be the current user to get A2DP proxy

After user switched, Bluetooth process will be restarted to run as
secondary user and some system process like AudioService keeps as
primary user. When BluetoothA2dp did rebind, it still used primary UID
for those system process and caused they could not invoke A2DP APIs.

This changes to use current user ID when rebinding to A2dpService and
system process can get correct A2DP proxy after user switched.

Bug: 112469503
Test: manual
Change-Id: If5ee945b0dcf2044ef2baedd762751fa6ef3d09a