gatt: Add upper tester for Multiple Variable Len notification

Core spec requires GATT Server implementations to support Multiple Variable
Len Notification for the certification purposes.
This patch adds simple Upper tester which helps to pass GATT/SR/GAN/BV-02-C.

Proper API for Android might come later if any.

To enable upper test two steps are required
1) set GATT_UPPER_TESTER_MULT_VARIABLE_LENGTH_NOTIF to TRUE
2) PTS_ForceEattForNotifications=true

< ACL Data TX: Handle 64 flags 0x00 dlen 19
      Channel: 84 len 15 sdu 13 [PSM 39 mode Enhanced Credit (0x81)] {chan 0}
      ATT: Handle Multiple Value Notification (0x23) len 12
        Handle: 0x009a
        Length: 0x0003
          Data: 797979
        Handle: 0x00a2
        Length: 0x0001
          Data: 02

Bug: 236702711
Tag: #feature
Test: atest BluetoothInstrumentationTests

Change-Id: I8a3c47ddae2b8e5487ef112b84888ea34d33a3b1