floss: Update syslog filtering for libbluetooth

Add functions to set the default log level and log level by tag to
replicate the log filtering functionality previously implemented via
init flags.

Bug: 290841136
Bug: 305066880
Test: ./build.py; mmm packages/modules/Bluetooth
Flag: Exempt, logging change
Change-Id: Ic200323daac41eaffb5fc2a40a36bb22a5f3ac2f