base

Fix unsafe concurrent access in LegacyTypeTracker

This patch adds synchronization inside LegacyTypeTracker so that
getNetworkForType() can safely run concurrently with remove().

Without synchronization if remove() removes the last network for a
given type while getNetworkForType() runs for the same type, it is
possible that getNetworkForType tries to access the head of an empty
list, resulting in a runtime exception.

This issue was found by zoran.jovanovic@sonymobile.com who proposed a
fix in AOSP (Change-Id: Ia963662edb9d643790e8d9439e4dbdcac4c2187b).

This patch differs from the fix proposed by the bug reporter and tries
instead to do the minimum amount of locking to make getNetworkForType
safe.

Bug: 29030387
Change-Id: I915aac527fc8828b32bf35fee870add2dfb11d8d