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
Merged-In: Id56757ba7d374f309615b8149ad37a7760e46651
Change-Id: Id56757ba7d374f309615b8149ad37a7760e46651
(cherry picked from commit 1b7644b504adb928a20df1e6c9405ab34eb7a468)