Make L2CAP credit config modifiable

Value is overridable by build time system properties:
  bluetooth.l2cap.le.credit_default.value
  bluetooth.l2cap.le.credit_threshold.value

Bug: 263323082
Bug: 174290912
Test: Manual test
(cherry picked from commit 5fa1707fd2d7ed5d040ab8aa2c35d07a3ce920fa)
(cherry picked from commit 4f221c97)
Merged-In: Ic27f572e73527bdf1af47e3781372043f8e310be
Change-Id: Iaced125567764ab5c9d373f10fea6417c589f3e0