A2DP: Workaround to clean up previous AVDTP instance while re-enabling

If the user disables and re-enables A2dpService quickly, the stack has
chances to be still busy at dropping those out-of-date links, and this
is highly dependent on the handset. If the handset responds more slowly, it
spends more time to disable, but causes the new AVDTP instance won't
work. This change checks the previous disabling flag, and cleans up the
old instance up before re-enabling.

Bug: 135655859
Test: manually
Change-Id: I625cfffab6d4df327fb8159aa4b759a09d90e076