eatt: Add upper tester to trigger Read Multiple Value Len Req

Android does not expose such API but to the used, but when EATT is
supported, Android needs to pass Read Multiple Value Len Request
qualification test. Note: It does not force us to expose this as API.

Upper tester is added for this purpose and should be enabled with
GATT_UPPER_TESTER_MULT_VARIABLE_LENGTH_READ

Bug: 236083555
Test: atest BluetoothInstrumentationTests
Test: GATT/CL/GAT/BV-03-C
Tag: #stability
Change-Id: Id56757ba7d374f309615b8149ad37a7760e46651