Change bluetooth cache to be static

Whith a non static cache, we are creating a cache every time a new
BluetoothDevice is created, and then we have to rely on the GC.
The number of cache can sometimes reach 10 000.

Fix: 235637412
Test: basic bt functionnality
Test: adb shell dumpsys cacheinfo com.android.bluetooth
Change-Id: I6659dd9050b42bb57d0640df8117150cd67cb026