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