Fix linux BLE deadlock

[tehampson]

Problem:

• After Assert the Matter lock is held in non-threadsafe SystemLayer methods. #26180 calling pychip_ble_start_scanning resulted in assert while calling StartTimer while in ChipDeviceScanner::StartScan. This resulted in chip-repl hits the Code is unsafe/racy assert when BLE commissioning is started #26338 PR being created
• Unfortunately chip-repl hits the Code is unsafe/racy assert when BLE commissioning is started #26338 introduced a deadlock for other callers of ChipDeviceScanner::StartScan. Those other callers call ChipDeviceScanner::StartScan from a method that was scheduled to run on the ChipStack with ScheduleWork.

Solution:

• The requirement can be that ChipDeviceScanner::StartScan is called with the ChipStackLock. These seems to allow both cases to coexist.
• The destructor calling CancelTimer still does seem to be dependent on if it was called in scheduled work that does call TimerExpiredCallback, or some other thread. For the time being acquiring the ChipStackLock logic over there still makes sense.

Test:

• chip-tool pairing ble-wifi 1 asdf asdf 20202021 3840, no longer deadlocks
• In chip-repl issuing asdf = chip.ble.DiscoverSync(timeoutMs=2000) followed by for x in asdf: print(f"TMsg {x}") works.
• In chip-repl, issuing devCtrl.CommissionWiFi(3840, 20202021, 1, "asdf", "asdf") no longer deadlocks.

[github-actions]

PR #26418: Size comparison from 6c0c5ea to f1c4ab1

Full report (1 build for cc32xx)

platform	target	config	section	6c0c5ea	f1c4ab1	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	605090	605090	0	0.0
			(read/write)	204164	204164	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197576	197576	0	0.0
			.comment	206	206	0	0.0
			.data	1468	1468	0	0.0
			.debug_abbrev	957621	957621	0	0.0
			.debug_aranges	101136	101136	0	0.0
			.debug_frame	341512	341512	0	0.0
			.debug_info	19609755	19609755	0	0.0
			.debug_line	2666632	2666632	0	0.0
			.debug_line_str	513	513	0	0.0
			.debug_loc	33340	33340	0	0.0
			.debug_loclists	1489225	1489225	0	0.0
			.debug_ranges	4984	4984	0	0.0
			.debug_rnglists	94315	94315	0	0.0
			.debug_str	3108982	3108982	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104354	104354	0	0.0
			.shstrtab	265	265	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	483384	483384	0	0.0
			.symtab	287328	287328	0	0.0
			.text	498612	498612	0	0.0

[bzbarsky-apple]

Thank you for hunting this down!