Use a system property to drive the maximum number of GATT connections.

A few notes:
- The maximum is capped at 16 regardless of the property. With static allocation
  and subsequent initialization of resources bluetooth will time out if this is set
  too high.
- The default (and minimum) max value is still 7.
- Other modules can have their own limitations. HID will only allow 14 LE connected
  devices.
- gatt_utils.cc references the property-driven value everywhere for consistency, but
  it's probably good enough to just do so in gatt_allocate_tcb_by_bdaddr

Bug: 283344957
Test: atest BumbleBluetoothTests
Flag: EXEMPT - changes in #define directive can't be guarded
Change-Id: I858d6f5ec9947031276b6a72ecbf6c5d50fbed1e