ErrorProne: DatabaseManager

```
84: warning: The field 'LEGACY_BTSNOOP_DEFAULT_MODE' is never read.
            LEGACY_BTSNOOP_DEFAULT_MODE = "bluetooth_btsnoop_default_mode";
            ^
127|144|151|157: warning: Synchronizing on non-final fields is not safe:
if the field is ever updated, different threads may end up locking on different objects.
                    synchronized (mDatabase) {
                                 ^
555: warning: Write to static variable should not be guarded by instance lock 'mMetadataCache'
            metadata.last_active_time = MetadataDatabase.sCurrentConnectionNumber++;
                                                        ^
1076: warning: The type `Map.Entry` was referred to by the non-canonical
name `HashMap.Entry`. This may be misleading.
        for (HashMap.Entry<String, Metadata> entry : mMetadataCache.entrySet()) {
                    ^
```

Bug: 236759221
Test: m RUN_ERROR_PRONE=true Bluetooth
Ignore-AOSP-First: Merge conflict resolution
Change-Id: I848452e9b1c03b08ee55c35ff286a3080b32ee73