PandoraServer: Remove hardcoded BT_OPP_LAUNCHER_ACTIVITY

The BT_PKG_NAME associated with it is only valid for
AOSP targets, internal targets with Google apex have a
`com.google.android.bluetooth` package.
So we remove the hardcoded value and instead use
PackageManager to retrieve it

Test: atest pts-bot:OPP/CL/GOEP/BC/BV-02-I on git_master
Bug: 270724641
Change-Id: Ibdc2c2f0c99ef2b6ec8602d844940264bbe974e1