Close BluetoothGatt when disconnected

To alleviate potential bugs from reusing BluetoothGatt
instance with opportunistic = true, close it and re-create
it when it's needed.

Tag: #feature
Bug: 233292528
Test: atest BluetoothInstrumentationTests and manually
checked the battery level

Change-Id: Ie1f32fc6f2bb96971760fff09a1dddd1fe87ab33