Unhide BluetoothGatt.requestSubrateMode

Enables device developers to control subrate parameters in order to optimize for power vs. latency.
Updated to generated framework/api/current.txt.

Bug: 384113935
Bug: 315241296
Test: m com.android.btservices
Change-Id: I6921f257e25a5ba31a9dc7d7c38eed4dcd664bed